In the Church Of Clean

Copied from here.
"Information copyrighted and courtesy of XJ4Ever Inc., Atlanta, GA."


NOTE1: unfortunately, the title above is somewhat mis-leading, for there are no real "tricks" to rebuilding these carb sets correctly…….just having the proper knowledge, the proper tools, and the organizational skills, persistance, and a dedication and determination to doing the job right! These things aren't magic. They aren't due to luck. Having your carbs perform properly is not akin to winning the lottery, i.e. some people get lucky and others don't and that's just life.

If you dig deep, if you could stand over the shoulder of those who are successful and those who aren't, the ways in which success is determined are pretty clear-cut and straight-forward, as outlined further below.

As are, unfortunately, the ways in which a lack of success are achieved.

This guide aims to help you achieve the former.

NOTE2: these guidelines are written specifically with the following carbs in mind:

  • the Hitachi HSC-32 and HSC-33 series carbs used on the XJ650 (except Turbo), XJ700 non-X (air-cooled) models, and all XJ750 models (except "X" models), and…..
  • the Mikuni BSxx series of carbs used on XJ550, XJ650 Turbo, XJ700-X, XJ750-X, XJ900RK, and XJ1100 models.

Although many of the same thoughts and priciples will apply to other similar carbs used on other Yamaha models, some minor (or major) differences may apply to other model bikes and carbs.

NOTE3: between research, reading, learning, studying, and then actually DOING the carb removal, rebuild, re-installation and tuning tasks, expect to spend a good 16-20 total hours during your first go-round. And at least 2 of those hours are going to be spent on simply removing and then re-installing the carbs back onto the bike, and then another 2 hours performing the after-install start-up, synchronization, and tuning efforts. If you don't leave yourself enough time to do the job correctly, you will end up rushing, taking shortcuts, and breaking Commandment #1 (further below), and typically horrible results and outcomes can be expected.

Take your time, be prepared, do it right, and it's a fairly simple process. Do it in any other manner, and you'll chance living the old Fram oil filter commercial warning and slogan:

"You can pay me now, or you can pay me (a lot more) later………"

NOTE4: after reading all what's written below, you might be immediately tempted to:

a) doze off to sleep.

b) go running, screaming, head in hands, to the nearest bar or pub.


c) go out and purchase guns and ammo.

RELAX! Although it seems long…..a lot longer than the 5-6 pages that the factory service manual chapter devotes to the entire "carb rebuild" topic….that's because it's more inclusive and includes all of the things that they "forgot" to include (or didn't even realize would be necessary after 25+ years). Service manuals are designed for experienced service technicians, and are designed to guide them through the complete process, but only with the most basic steps necessary to accomplish the job while also beating the "flat-rate manual" time allotted for the task.

What is detailed below is basically not only a more (much more!) exhaustive and thorough "rebuild" procedure than the factory would ever dream of specifying, but they're also written in a manner to help guide a non-experienced wrencher thru the process CORRECTLY the first time around, and will explain not only the "basic service", but also how to almost "blueprint" your carbs to good-as or better-than-new condition.

So break out your favorite beverage and spend a little time reviewing this info. At the bottom of this section are a few great links to some pictorial guides on both the Hitachi and Mikuni carbs, that, when used along with the suggestions below, may allow you to visualize all the dirty little details mentioned below much more easily.

HOWEVER, as we mention elsewhere, this is not a step-by-step instruction manual! So please make sure that you also have available a factory or aftermarket workshop manual to guide you along your journey to carburetor Nirvana (there will be 4 dancing, almost-virgin carbs waiting for you there………).

Good luck! But, with what you now have at your disposal, luck will perhaps not play even a minor role anymore in the success of your carb rebuild project!


Introduction: A Few Thoughts We Hold to be Not-So-Self-Evident

a) If you don't plan on checking and correcting (if needed) the valve shim clearances, then don't bother cleaning and rebuilding the carbs.

b) If you don't plan on synching the engine when you're done, why bother cleaning and rebuilding the carbs?

c) If you don't plan on using a colortune plug to properly tune the carbs when you're done, what difference does it make whether they're clean and rebuilt?

d) You will never, Ever find anyone who is willing to do as thorough a job in cleaning your carbs as you are.

e) Probably a good 50% of the posts on these forums (and others) revolve around the subject of "My bike doesn't run right." or "won't idle correctly" or "it won't start when cold" or "it idles fast" or…….well, you get the picture. Almost all of these issues are carb-related issues, and their causes, although many and varied, all boil down to one thing:

Your carbs are out of adjustment, internally dirty (probably filthy, actually), and in critical need of not just "cleaning", but of REBUILDING. You might want to consider that CLEANING the carbs—inside and out—is just one of the many steps (a very imporant one, for sure) in the overall process of REBUILDING the carbs.

The Gospel of Clean:

But all of the above is actually the good news! Because now you've isolated the performance problems down to one specific component.

But that's also the bad news. Because a proper rebuild and re-tuning of these carbs, while not mechanically difficult, can hold quite a few surprises for the un-experienced, the un-wary, and especially for the lazy or sloppy rebuilder.

Our basic scripture in regards to carb rebuilding (or any other project, for that matter) is this:

Do it once, completely, thoroughly, and correctly.

Because: that's how you save yourself money, time, frustration, and aggravation.

Today's Sermon:

To that end, I offer you this short sermon titled CARB REBUILDING GOTCH-YA'S in order to make your carb rebuilding experience easier, sexier, to take some of the mystery out of the process, and hopefully allowing you to be more fully prepared to tackle AND TO SUCCEED at the task at hand.

NOTE PLEASE NOTE: what follows is not, nor is it intended to be, a complete step-by-step, blow-by-blow guide to carb rebuilding. It is intended to be a "helpful assistant" which may help you prevent making the most common mistakes that you will most likely make during your first attempt at this project! Think of it as a "Supplementary Guide" which will try to give you some of the details that Yamaha and Haynes left out of their typical non-specific "assembly is the reverse of dis-assembly" type of instructions when it comes to the carbs.

More complete step-by-step procedures can be found in other posts in these forums and in the factory and aftermarket service manuals.

The At-The-Pearly-Gates Question: To Break or Not-To-Break?:

While many people claim that dis-assembling the carbs from the rack during the rebuild process is NOT necessary, we'll respectfully disagree.

Although that statement may be true in a few specific instances, it is my opinion that such a course of action is "false economy"—-in return for less effort now, you're going to be sitting on ticking time-bomb of future problems just waiting to happen, and waiting to be mis-diagnosed ("hey, I already cleaned the carbs and still have these problems, what now?"), and thus additional time & expense.

The devil normally resides in all of the details, right?

"Breaking the rack apart" means taking off the 8 upper and 8 lower rack screws, and then gently "prising" (as the Haynes manual likes to say) the carb bodies apart from each other…….once the rack bracket screws (upper & lower) are removed, the only thing holding the bodies together are:

  • the fuel supply rail connectors (these have o-rings on their ends and fit into the fuel supply bore holes in the carb bodies).
  • the starter enrichment ("choke") circuit pivot rod and their carb bracket attaching set screws.
  • the "overlap - interplay" of the throttle shaft arms between one carb and the next…..look carefully at an assembled rack (where the SYNCH ADJUSTING SCREWS are) and you'll see how the lever arm of each carb's throttle shaft "intercourses" with the corresponding one on the adjacent carb(s) (note that one carb does not have this overlap).

This overlap between the throttle arm brackets is not a mechanical connection; it is just that, an overlap. You "disconnect" one carb from the other by gently pulling them apart from each other, with some twisting action normally required to get fuel supply connector tubes to break loose from one or the other carb body fuel tube bore.

That's it! Takes about 10 minutes total additional time to do. Be aware that the factory used some type of thread locking compound on the rack screws so they may need some persuasion to get them moving.

Our experience has led us to the conclusion that not fully rebuilding these somewhat finicky carb sets while you have them off the bike—and lets face it, the removal and re-install process is one of the most time-consuming and tricky parts of the whole process, and has to occur no matter how much or how little "rebuilding" goes into the carbs while they're off the bike!—is not the correct way to proceed.

Atonement: If You Break, You Must Then Repent and Un-Break Your Rack, Too:

After you finish you carb rebuilding procedures, you have to put Humpty-Dumpty all back together again, and the major goal of this part of the process—the one that seems to scare most people off-is getting the carbs back together IN PROPER ALIGNMENT with each other…….but, it really isn't tricky to do, at all. The key is to simply hook all four carbs back up to each other—meaning the fuel supply tube connector pipes and the "overlap" between the synch screw arms (and trust me, it's easier to do than it looks!)—BEFORE you start to install the upper or lower rack brackets and their screws.

You'll then take this un-bracketed "rack" of carbs and put them, intake-manifold "throat" face-down, onto a REALLY, ALMOST-PERFECTLY FLAT piece of "something". The factory manual recommends using a piece of plate glass, or you can purchase our specially made rack alignment plate produced specifically for this job.

You'll also discover that putting a board (a 1 x 2, etc.) across the top of the carb intake ports (the airbox "throats", which will now be facing straight towards the ceiling since the intake-manifold throats are against the alignment plate), which will allow you to put some hand pressure on the carbs, which is necessary to keep the carb throats in constant and complete contact with the alignment plate as you are installing and tightening the screws. KEEPING THE CARB INTAKE-MANIFOLD THROATS IN FLAT, CONSTANT CONTACT WITH THE ALIGNEMNET PLATE IS WHAT "INSURING PROPER CARB ALIGNMENT" WHILE TIGHTENING THE RACK BRACKET SCREWS IS ALL ABOUT. Believe me, it's not rocket science!

The individual carbs will automatically align themselves properly in their side-to-side alignment as the rack bracket screws are introduced—-that is insured by the drilled holes in the carb bodies aligning with the drilled holes in the rack bracket. It's the perfect front-to-back alignment or slight "twisting" of the carb throttle shaft overplay that might cause the front (or rear) spatial mis-alignment, and that's the purpose of the alignment plate……to make sure that the carbs stay aligned in ALL their dimensions as their brackets and screws are re-installed and tightened.

Otherwise, if the carbs do not stay in such alignment, the throttle shafts and thus, the throttle (butterfly) valves will never be truly aligned, and this situation will cause you untold amounts of grief down the road.

As with any other multi-fastener installation procedure, tighten each screw a bit at a time, maybe in an odd-even pattern, first the top bracket screws, then the bottom bracket screws, then back to the top, etc.

Remember to use some semi-permanent thread locking fluid on both the upper and lower rack screws.

If you're saying to yourself by now—"gee, that sounds pretty simple"—well, you're right! The devil is in the little details as mentioned elsewhere, mostly pre-prep work and the marking of the component pieces. The re-alignment of the carbs back into a completed "rack" is easy-cheesy! Believe me, the mechanical "nuts and bolts" aspects of breaking the rack apart are SIMPLE; it's mostly the meticulous attention to the small details BEFORE YOU EVEN START that holds the keys to success or the trap of failure for this project.

Those 10-20 minutes of inspection and notations you made before you break the rack or the individual carbs apart takes care of that issue……..things like, note the way the throttle arm brackets overlap. Note the way the throttle return springs are located on their stop pegs. Note where the hose and cable clips are located, and in which direction they are oriented, etc. Those kind of details.


So please, consider doing a Full and Complete Carb Rebuild procedure when you have the time. Given the age of the bikes, the lack of attention that the previous owner paid to them, and the sloppy level of work that dealership service departments do (or may have done in years past)——give your carbs the proper attention that they deserve. You'll learn alot, you'll solve most of the carb related problems with these bikes, and you'll have the pride, knowledge, and experience of having done it yourself.

And your carbs will thank you………

Genesis, a/k/a IN THE BEGINNING……..:

Have ALL parts and supplies you can even imagine or consider replacing already ordered and received.

- at the end of this guide there is a semi-complete list of parts that you'll need, besides any that you discover to be missing, damaged, or unusable.

Have ALL of the tools, special or common, that you'll need. Stopping and starting not only wastes time and effort and is frustrating, but it leads to confusion or shortcuts, both of which are 4-letter words in the carb-rebuilding biz….

- at the end of this guide there is a semi-complete list of tools that you'll need.

Obtain (beg, borrow, but do not ever Steal) a Factory Service Manual and/or an aftermarket Haynes or Clymer Owners Workshop Manual and read through it before you begin. Try to make sense of the pictures and procedures IN YOUR MIND before you even remove the carbs.

Especially if this is the first time you've attempted a carb rebuild, this "pre-learning" will most likely be invaluable training for you.

Have a pad of paper and a pencil and take AT LEAST 5 pages of notes, sketches, and diagrams. That's five 8-1/2" x 11" pages. If you don't fill up five pages, you have either:

a) done this at least 10 times before, or…..
b) you're going to hate yourself in about 2 days time.

Better yet, have a digital camera and a friend available for at least the entire 2 hours carb rack removal and dis-assembly steps. Count on taking at least 100 pictures.

Think I'm joking? I'll place 10-to-1 odds that you'll be cursing like a drunken sailor AND THROWING THINGS within 36 hours if you don't take the above advice!

Exodus, a/k/a PRE-DISASSEMBLY:

NOTE: Memorize this!: the #1 carb is the left-most carb, as you're sitting on the bike (clutch lever side of the bike). #4 carb is the right-most carb (brake lever side of the bike). Carbs are "numbered" starting with the #1 (far-left) carb and then progressing, left-to-right, to #2, #3, and finally #4 (the far-right carb).

Whether you're just taking the carbs partially apart, or doing a full atomic-level rebuild, it's important to remember that while some parts—such as the carb hats, screws, bowls, floats, jets, etc.—are interchangeable parts, it's still a great habit to keep them separated , since you'll eventually get around those parts that are NOT interchangeable between carbs….and if you don't take note of them BEFORE you take the individual carbs apart, then you will get confused upon re-assembly (actually, you'll eventually figure it out, but trust me, it's easier to do the 10 minutes of advance prep, study, and making notes beforehand).

Here's some helpful tips as you dive into your dis-assembly:

  • the Carb Bodies…mark them 1, 2, 3, 4, with an engraving pen or a point punch. Although some carb bodies are marked from the factory with their number (i.e. 1,2,3,4) stamped into a flat flange surface on the body, some aren't. And if not, you'll need to mark them with some method to insure that you get it right, since:

a) the butterfly shafts and valves MUST to be returned to their original bodies, and…..

b) although you can tell carb bodies #1 and #4 apart from each other even if they are not marked, on some models carb bodies #2 and #3 are identical to each other but their throttle shafts and valves need to go back into their correct bodies…….and this is why you mark the carb bodies 1, 2, 3, 4.

You can mark them with a point punch (1 dot per carb body number), or an electric engraving pen, or a set of number punches and just whack them onto the flat surface near the lower carb mount boss.

  • Mark the throttle shaft - arm units with an engraving pen 1, 2, 3, 4………there are actually three similar-but-different arms, and you want to make sure you assemble the correct shaft back onto its proper carb body!
  • Mark the removable throttle shaft end brackets with an engraving pen 1, 2, 3, 4…….there are actually three similar-but-different arms, and you want to make sure you assemble the correct one back onto its proper carb body!
  • Butterfly valves….mark them 1, 2, 3, 4 on the side facing the airbox with an engraving pen. Scribe an arrow so that you'll know which way is pointing up or down (doesn't matter which, as long as you're consistent) so that the butterflies can go back into their proper carbs, and oriented the same way they came out. If you don't get this right, expect big troubles later!
  • Carb bowls, there are left-side (#1 or #2) and right-side (#3 or #4) bowls. No need to number them, as it is obvious which is which, but just remember which are which when doing re-assembly.
  • Make some type of mark on one end and write it down (#1 end or #4 end) of the starter circuit rod (use a small saw mark or point punch one end).

By the way, have I mentioned how handy an electric engraving pen ($10 or so at Wally World) is when working on these carbs?

  • The tightly-wound throttle return springs…….mark them or bag them individually, there's 3 different ones. Note where and how the spring end hooks are oriented in relation to the throttle arms and the carb body retaining tangs before taking them off the carbs! Make a sketch! You'll never remember this later!!!!
  • The various cable and hose Routing Brackets and Clips on carb hats and on the upper rack bar……..make a diagram of which one goes where, and how - which way it is oriented when installed.

40 Minutes in the Desert, a/k/a/ DIS-ASSEMBLY:

This is the easy part! But make absolutely, positively sure that you take as many notes and diagrams as you need NOW, so that you'll remember all the important "little things" LATER in regards to what order in which the parts come apart, where they are located, what their orientation is, etc.

Although it is more time-efficient to take all the carbs apart all at once, and then move onto the cleaning and rebuilding tasks and do all of that as a "batch", if this is your first Carb Rodeo we suggest you take one carb completely apart first, make your notes, etc., and then immediately (skip the cleaning, etc. tasks for now) put it all back together again.

In this manner, you'll quickly learn the things that you overlooked or "forgot" to make notes about, and you'll still have another (actually, three "others") carbs to use as a true reference point to compare against. EVEN IF YOU HAVE LIMITED MECHANICAL SKILLS OR PREVIOUS CARB KNOWLEDGE, THIS METHOD PROVIDES A FAIL-SAFE WAY FOR THE 100% SUCCESSFUL COMPLETION OF A CARB REBUILD PROCESS. This step will add maybe an additional 30 minutes to the overall total rebuild process, but might save you an incredible amount of time and grief later………

Here are some problems areas that you should pay special attention to, and take extra precautions with:

- the Carb Hat Screws………on Hitachi model carbs, there are different lengths in different positions. Almost all positions that have a clip or bracket associated with them use longer screws.

Mikuni models use the same length carb hat screws in all positions.

- the removal of the Idle Mixture Screws………this can be tricky on two fronts:

  • the screws were originally covered with an anti-tamper cap, and this cap may or may not still be in place. If it is, then you must CAREFULLY remove it by drilling a small hole in the center of it (perhaps 1/8" diameter, using a slow-speed drill) until you just barely puncture through this cap. Careful!——for the head of the idle mixture screw is directly below this cap!!!

Now, take an appropriate sized sheet-metal, drywall, or wood screw and drive it into the hole you just drilled, and then yank the cap out of it's hole. The cap is just a "friction-fit" into this hole, so it should come out "kinda" easily……

Now you have to remove the idle mixture screw itself: these are commonly seized, and you do not want to ruin the head of this screw while trying to remove it. We offer a properly-sized idle mixture screw Screwdriver (HCP254) that fits the slot head of this mixture screw perfectly, greatly aiding in the safe removal of this screw (it's also great for performing the future mixture screw adjustments that you'll need to perform later as part of your carb tune-ups).

To assist in the removal of this screw, use all the tricks at your disposal for extracting seized or frozen fasteners: heat, penetrating fluids, a blow to the head, a slight "tightening" force applied before attempting to loosen it.

If the head of these screws are already (or become) damaged, then it's onto Drill Bit and E-Z Out World for some fastener surgery. IF YOU DON'T FEEL CONFIDENT PERFORMING SUCH A TASK, THEN TAKE THE CARB BODY TO A MACHINE SHOP AND PAY THEM TO EXTRACT IT. This will be alot cheaper than having to replace the carb body if, by mistake, you drill out or otherwise destroy the threads in this area!

Once the screw is removed, be sure that the tension spring Washer and the tiny O-Ring at the bottom of the bore also come out….the o-ring, especially, can get stuck at the bottom of this bore. Use an appropriate sized bent-tip dental pick to manuever this o-ring out.

- the Retaining Screws that hold the butterfly valve to the throttle shafts……on Hitachi carbs, they use self-locking threads and the heads strip easily. Make sure you use a proper sized screwdriver to try and remove these screws or you'll end up with a real mess to resolve!!

On Mikuni carbs, this story goes from merely bad to worse: these screws are an absolute BEAST to remove, and it is very easy to destroy the screw heads as well as the throttle shaft threads while trying to remove them—-if proper precautions are not taken. You will want to review the information in this forum thread before attempting removal:…html#66348

All other fasteners, be they bolts, screws, or nuts: they're not self locking, but they strip even easier, especially if you do not use the proper size and type screwdrivers.

Float Pin Removal Warning: failure to heed this advice can send you straight to carb purgatory, so read carefully:

The FLOATS are "pinned" in place via a small-diameter steel FLOAT PIN, in-between two cast upright "arms" that extend up from the carb body floor. SUFFICE IT TO SAY THAT THESE CAST FLOAT PIN SUPPORT "ARMS" ARE MORE THAN EXTREMEMLY FRAGILE! Break one of them off, and the carb body is toast……

On Hitachi model carbs, the float pin is a straight pin, and is slightly under-sized compared to the size of the pin holes in these cast support arms……and as such, they are usually pretty easy to remove (to remove the floats and the float needle valve assembly, this float pin must come out first). Sometimes corrision and gunk can cause this pin to seize, in which case you must be very careful in your attempts to remove it. The careful application of some heat to the cast arms, in the pin hole area, will cause the slight expansion of the cast aluminum area surrounding the float pin, and can help free a stuck pin.

On Mikuni models, you're in a world-'o-hurt before you even begin. The float pins on all Mikuni carbs are not undersized, and almost seem as if they were designed to be a "fiction fit" within the pin holes in those cast support arms……and as such, they are nearly impossible to remove easily. In addition, the Mikuni pins have a mushroomed "head" on one end, meaning they can only come out in one direction, which can present some challenges all their own if you have not broken the carbs out of their rack……

Once again, they key is patience, penetrating oil, heat, and gentle pushing - punching on the end of the pin. IT IS INCREDIBLY EASY TO BREAK THE CAST UPRIGHT SUPPORT ARMS ON MIKUNI CARBS WHILE ATTEMPTING TO EXTRACT THESE FLOAT PINS!

So……….IF YOU DON'T FEEL CONFIDENT PERFORMING THIS TASK, THEN TAKE THE CARB BODY TO A MACHINE SHOP AND PAY THEM TO EXTRACT THEM. This will be alot cheaper than having to replace the carb body if you break off these support arms!

Once the float pins are driven out (using a proper sized punch) even slightly, you can grasp the exposed end of the pin with some pliers and carefully "twist-and-pull" the pin the rest of the way out. Make sure you twist and don't just "pull"!!

And once they're out, please do polish the pin with some very fine finishing paper and remove any deformities that the punch, or the pliers, may have caused on the end(s) of the pin.

Starter Plunger Valve Removal Warning:

This brass starter (enrichment) circuit plunger valves are actuated by stamped steel "finger" brackets which ride on the rotating starter circuit shaft. The "finger" bracket grabs this brass valve at the "head" of the valve, in a recessed area of the starter valve shaft. This machined-down "recessed' area of the plunger valve is a weak point in the valve, and if the plunger is stuck in it's bore, you can easily snap off this head at that recessed point. This problem is most common on Mikuni carbs, but it applies to all the carbs. Be afraid, be Very Afraid!, of any starter plunger valve that is seized in the bore when you try to remove it!

Also——on some Mikuni models, the needle tip of these starter plunger valves are embedded into the base of the plunger via a rubberized type seal, which will melt and be destroyed if immersed in carb dip. And no, the seal is not replacable, you'll need a new plunger valve…………

You're Screwed…….Or, All About Fasteners:

  • Some very useful tips for removing screws is the following:

- always select the proper sized screwdriver for the screw head! ALL phillips-drive screws on these carbs—in fact, on the entire bike-are what are known as "JIS" type screws. JIS stands for Japanese Industrial Standards, and similar to our SAE specifications, the Japanese have their own style and specifications for various hardware and fasteners, and one of those has to do with the type of "phillips slot profile" used in screw heads. It's different than in the rest of the world and to properly remove these screws, you need to use—you guessed it!—JIS profile SCREWDRIVERS.

- begin every screw removal task by placing the proper sized screwdriver tip into the screw head, and then giving a quick, sharp, forceful blow to the screwdriver head with hammer (or similar)……in essence, as if you were trying to "hammer" the screw, like a nail, into the object. This force goes a long way towards breaking the physical bond between the screw threads and the object it is screwed into, thus making screw removal so much easier and safer.

- next, apply a slight TIGHTENING torque to the fastener before you attempt to loosen the screw, bolt, etc. "Slight" is the key word here! You are not actually trying to tighten the fastener any more than it already is, but you are trying, again, to break the physical bond—ever so slightly—between the threads of the fastener and the object it is fastened to. Even the tiniest bit of movement or force can have this result, which then makes the removal process that much easier and safer.

- when encountering frozen fasteners, the use of HEAT and COLD when trying to remove them is a much better idea than simply trying to use brute force. Penetrating oils of various kinds (hint: olive oil is a great lubricant, and is basically non-flammable) works better if the part is heated. The differential expansion coefficients of dis-similar materials, including those seized-together metal parts, is a natural law of God and the Universe and is free to everyone. By HEAT, we mean a heat gun or a pencil tip torch, used very carefully (less is more). By COLD, we mean like 2-3 hours in the freezer or immersed in ice chips or water.

- another useful trick is to heat as much of the area surrounding the stuck fastener…..but not the fastener itself…..and then take a crayon or a wax candle and apply it to the very joint of the fastener and its threaded bore hole. The wax will be wicked into the threads of the fastener and its surrounding hole. Then let it cool down to the touch and repeat twice this process twice more. The wax acts as a lubricant, and follows the heat——so put the heat where you want the wax to go. You'll be amazed…….

- sometimes, even after Heruclean efforts, you just ain't gonna budge (or you've broken off) that puppy. Then, the world of ez-outs, left-handed (counter-clockwise rotation) drill bits, and stud extractors (extremely useful on snapped-off intake manifold bolts or exhaust flange studs) will become your best friends.

- after fastener removal, if you encounter threads that are stripped or corroded or buggered or anything other than "pristine", you need to "clean" or re-cut the threads back to their original condition (typically called "chasing" the threads). If the thread damage occurs due to corrupted threads on the fastener itself, then you might be able to "chase" the threads back into good condition (the thread in the carb body, that is!) by using a known good screw (of the same size and thread pitch, of course) to perform this "re-threading" operation (after all, the screws are steel, and the carb bodies are aluminum, so the steel threads of the screws will tend to "cut" new threads easily in the softer aluminum of the carb body). A better way is to use a tool-steel re-threading TAP tool, as these are both harder and more precisely made than a regular screw, and are the professional method of cleaning, chasing, and/or re-cutting new threads.

The following goes without saying, which means we need to say it anyway: Do not EVER attempt to re-use a fastener that has damaged threads by forcing it into a threaded hole!! And this warning is especially true for soft-metal objects, such as carb bodies, as the (damaged) harder steel screws will cut new, now damaged threads into the softer carb body metal.

Everything else is simply cleaning and screwing things together, and if you can tell apart the business-end of a screwdriver from the handle, there's not that much more necessary in the way of "special skills" needed to rebuild carbs!

Towards The Promised Land: INSPECTION:

- Check the Vacuum Piston Rubber Diaphram for even the tiniest of air holes or tears. Hold it up to a strong light and if you can see light (pin holes) in the diaphram, it may be toast. One method of repair is to use an ever-so-thin smear of the "liquid electrical tape" product to rebuild the diaphram surface, and thus block those pinholes.

Note that a vacuum diaphram that does not seat completely (or that has tiny pin-holes in it) will not hold the vacuum signal as strongly as others will, and thus that vacuum piston and the main fuel needle that it controls will REACT DIFFERENTLY (more slowly) to the vacuum signal in that carb as compared to the others. This can be a big problem! Make sure that the vacuum diaphram is not truly damaged, and make sure that it can re-seat itself into that little channel properly.

Additionally, there are plastic cinch rings both above and below the inner edges of the vacuum diaphram; these rings exert pressure against the diaphram, both locking it in place and providing a true vacuum seal between the diaphram and the outer diameter of the piston. On Hitachi models only, the top ring tends to develop a split, or even a complete fracture, over time (we've never seen a Mikuni cinch ring split, ever). As long as the rubber diapham material is no pinched or split in the vicinity of the fractured area of the cinch ring, then things will "probably" be okay.

If there is damage to the diaphram, then the entire piston and diaphram unit should be replaced.

Diaphrams that are in otherwise good condition could benefit from the application of our rubber Rnr Fluid, which is a specially formulated "rejuvenator" fluid that helps lubricate and keep rubber products soft and pliable.

- Check the long, skinny aluminum Main Jet NEEDLE to make sure they are the correct size, and also to make sure that they haven't retracted from their plastic top cap…..if so, carefully push the needle cap back "flush" with the ridge near the bottom of the cap. Also check the very end of the tip for scratches, gouges, or deformation ("mushrooming") of the very bottom, pointed tip.

Also check to make sure that all of the needles are of the correct and same size. The needle size is engraved into the needle barrell, just below that plastic top cap:

Hitachi models:

all USA 1980-3 XJ650 Maxim, Midnight Maxim, and 1982 XJ650RJ Seca models, all Canadian 1982-84 XJ650 Maxim models, and 1982-84 XJ650 UK/European 14R models:

- Hitachi HSC32 carbs
- Needle: Y-10
- Emulsion Tube/Needle Jet: 3.2mm

all Canadian 1980-1 XJ650 Maxim, Midnight Maxim, and 1982 XJ650RJ and RJC Seca models:

- Hitachi HSC32 carbs
- Needle: Y-12
- Emulsion Tube/Needle Jet: 3.2mm

all 1980-81 XJ650 UK/European 4K0 and 4K1 models:

- Hitachi HSC32 carbs
- Needle: Y-11
- Emulsion Tube/Needle Jet: 3.2mm

1981-83 all XJ750 (USA) models:

- Hitachi HSC32 carbs
- Needle: Y-13
- Emulsion Tube/Needle Jet: 3.2mm

1981-83 all XJ750 (Canadian) models and 1981-84 XJ750 UK/European (11M) models:

- Hitachi HSC32 carbs
- Needle: Y-14
- Emulsion Tube/Needle Jet: 3.2mm

XJ750 E-II models:

- Hitachi HSC33 carbs
- Needle: Y-17
- Emulsion Tube/Needle Jet: 3.2mm

XJ750RL models:

- Hitachi HSC33 carbs
- Needle: Y-18
- Emulsion Tube/Needle Jet: 3.2mm

XJ700 N/NC/S/SC models:

- Hitachi HSC33 carbs
- Needle: Y-20
- Emulsion Tube/Needle Jet: 3.2mm

Mikuni models:

XJ550 1981-83 Maxim:

- Mikuni model BS28 carbs
- Needle: 4GN
- Emulsion Tube/Needle Jet: O-8

XJ550 North American Seca:

- Mikuni model BS28 carbs
- Needle: 4GZ11
- Emulsion Tube/Needle Jet: O-8

XJ550L Maxim 1984:

- Mikuni model BS32 carbs
- Needle: 4CP7-3 on cylinder #2
- Needle: 4CP3-3 on cylinders #1, 3, and 4
- Emulsion Tube/Needle Jet: N-8

XJ650 Turbo:
- Mikuni model BS30 carbs
- Needle: 4DPS39
- Emulsion Tube/Needle Jet: O-6 (style #318)

- Mikuni model BS33 carbs
- Needle: 5FZ83 (XJ700 XNC models)
- Needle: 5FZ82 (all models except XJ700 XNC)
- Emulsion Tube/Needle Jet: Y-2

- Mikuni model BS33 carbs
- Needle: 5FZ83
- Emulsion Tube/Needle Jet: Y-2

XJ900RK, RL, and XJ900P models:
- Mikuni model BS35 carbs
- Needle: 4HZ22 (900RK) or 4HZ26-3 (900RL and Police)
- Emulsion Tube/Needle Jet: Y-0 (style #318)

XJ900 58L, 2HL, 3NG1, 3NG2 models:
- Mikuni model BS36 carbs
- Needle: 5FZ62-3
- Emulsion Tube/Needle Jet: Y-0 (style #318)

XJ900 1991-94 4BB1, 4BB2 models:
- Mikuni model BS36 carbs
- Needle: 5FZ10-3
- Emulsion Tube/Needle Jet: Y-0 (style #318)

- Mikuni model BS34 carbs
- Needle: 5GLZ-34
- Emulsion Tube/Needle Jet: X-2

BENNY AND THE JETS, a/k/a make sure your carb has the correct sized jets and note the positions of "which-jet-goes-where":

The fuel jets are always located in the bottom (base) of the carb body.

The air jets are located either on the upper side of the carb body…..underneath the vacuum piston diaphram….or, on some of the Mikuni models, in the carb throat, facing the airbox.

NOTE: on Hitachi model carbs, the main fuel jet and the pilot fuel jet are the same size and shape…..they differ only in the size of the passage drilled in them. On Mikuni carbs, the main fuel jet has the large, "mushroom" head, while the pilot fuel jet is a long, skinny "tube" style jet.


- the MAIN FUEL JET uses a copper washer underneath it, and screws directly into the removable "main needle jet" or "power valve" or "emulsion tube", and is the higher numbered (larger opening) jet. The jet size is stamped into the top face "rim" of the jets on all model carbs.


#102 XJ750E-II models (cylinders #2 & #3)
#105 XJ750E-II models (cylinders #1 & #4)
#106 XJ750RL Seca models.
#107 XJ700 non-X models.
#110 all XJ650 North American models (except 1982-84 Canadian models) and 1980-82 XJ650 UK/European (4K0) models
#112 all 1982-84 XJ650 UK/European (11N and 14R) models, and 1982-84 XJ650 Canadian models.
#120 all other XJ750 North American and UK/European models except XJ750RL models.


#100 1985 all XJ700-X models*
1991-94 XJ900
#102.5 XJ900RK US & Canada
XJ900RL and XJ900P (31A models)
XJ550L (1984) on cylinders #3 and #4
#105 1986 all XJ700-X models and all XJ750-X
XJ550L (1984) on cylinders #1 and #2
#107.5 XJ900 most 1983-90 except USA or Canada
#112.5 XJ550 all models and XJ1100
#125 XJ650 Turbo carbs for all cylinders
#127.5 XJ650 Turbo carbs for cylinders #1 and #4

#132.5 XJ650 Turbo carbs for cylinders #2 and #3**


  • there are discrepancies between the parts manuals and the service manual information on the size of the 1985 XJ700-X main fuel jets; one source claims these carbs were equipped with #100 main fuel jets, while other sources claim a #105 size main fuel jet.

** there are discrepancies between the parts manuals and the service manual information on the size of the 1982-83 XJ650 Turbo main fuel jets; one source claims these carbs were equipped with #127.5/#132.5 combination of main fuel jets as described above, while other sources claim #125 size main fuel jets were used in all cylinders. Original carbs that we have explored have the #127.5/#132.5 combination of jets as shown above.

- the PILOT FUEL JET has no washer and is the smaller (lower numbered) jet. Also check to make sure that all of the jets are of the correct and of the same size. The jet size is stamped into the top face "rim" of the jet on Hitachi models, and engraved into the side of the "tube" section on Mikuni models:


#36.5 all XJ700 non-X models.
#40 all 1980-83 XJ650 USA models, 1980-81 XJ650 Canadian models, 1982 XJ650RJ and RJC Seca Canadian models, 1980-82 XJ650 UK/European (4K0) models, 1981-83 all XJ750 USA models, XJ750E-II models, and XJ750RL** models.
#43 1982-84 XJ650 Canadian models, 1982-84 XJ650 UK/European (11N and 14R) models, 1981-83 XJ750 Maxim, Midnight Maxim Canadian models, and 1981-84 XJ750 UK/European (11M) models.

** there are discrepancies between the parts manuals and the service manual information on the size of the 1984 XJ750RL pilot fuel jets; one source claims these carbs were equipped with #40 pilot fuel jets, while other sources claim a #41 size pilot fuel jet.


Note that these Mikuni pilot fuel jets are recessed wa-a-a-a-a-y up inside the pilot fuel passage boss that is cast into the bottom of the carb body. In order to remove these jets, which are (of course) normally partially or fully seized, you must use a proper-sized flat blade screwdriver that will grasp and turn the pilot jet, rather than wallow around and chew up the slot in the head……which will then require tedious and dangerous surgical procedures involving ez-outs and the like to try to extract the jet without damaging the threads in that recessed, blind passge.

#32.5 XJ650LK Turbo (1983)
#35 XJ550 all 1981-83 models, XJ650LJ Turbo (1982), and XJ700-X** all models
#37.5 XJ750-X and XJ900RK USA & Canada only
#40 XJ550L (1984) model, all XJ900 except USA or Canada
#47.5 XJ1100

** there are discrepancies between the parts manuals and the service manual information on the size of the 1985 XJ700-X pilot fuel jets; one source claims these carbs were equipped with #35 main fuel jets, while other sources claim a #37.5 size main fuel jet.


- on Hitachi carbs, both the Main & Pilot AIR JETS are in the same size bores up top. The main air jet is in the "center" hole and is the lower numbered (smaller opening) jet.

The pilot air jet is in the hole closest to the "front" of the carbs, front meaning nearest the cylinder head, and is the higher numbered (larger opening) jet.

The rearmost hole is for the cover plate hold-down screw, if so equipped (some model carbs did not use the cover plate nor the
hold-down screw).

- on Mikuni carbs, the pilot air jet is located either under the diaphram cover, or on the bigger engines (700cc and up), in the carb throat, facing the airbox. The main air jet is always located in the carb throat, facing the airbox.


#50 all XJ650 models.
#70 all XJ700 non-X models, XJ750RL Seca models, and XJ750E-II models.
#80 all 1981-83 XJ750 North American models and 1981-84 XJ750 UK/European (11M) models.


All Mikuni BSxx series carbs do not have replaceable main jets. The main air jets on these models are in the carb throat, facing the airbox, and are a small, brass pressed-in tube that is not replaceable.

But just in case anyone is wondering, here are the sizes of these non-removable main air jets in the various models:

#45 XJ900 all
#70 XJ550 all models and XJ650 Turbo models.
#120 XJ700-X all models and XJ750-X all models*
#140 XJ1100 models*

NOTE: we believe that the #120 and #140 sizes listed in the service manuals (from where the above information was gathered) may be incorrect, as these are gigantic sizes for main air jets. But since you can't replace them anyway, I suppose that it doesn't really matter……..


#195 1980-81 all XJ650 North American models, all 1982 XJ650RJ and RJC Seca, and 1980-81 XJ650 UK/Eurpoean (4K0 and 4K1) models.
#205 1982-84 XJ650 Maxim North American models, 1982-84 XJ650 UK/European (11N) models, 1981-83 XJ750 Canadian models, and 1981-84 XJ750 UK/European (11M) models
#210 all XJ700 non-X models.
#225 all 1982 XJ650 UK/European (14R) models, all 1981-83 XJ750 USA models, XJ750E-II models, and XJ750RL models.


Pilot air jets on the Mikuni BSxx series carbs are replaceable and are located under the diaphram cover on all XJ550 models and XJ650 Turbo models, and in the carb throat, facing the airbox, on all XJ700-X, XJ750-X, XJ900RK, and XJ1100 models.

#140 XJ700-X and XJ750-X models
#155 XJ550L (1984) models, 1983 XJ650 Turbo* models
#160 XJ900 all models
#165 XJ550 Maxim models (1981-83), 1982 XJ650 Turbo* models
#170 XJ550 Seca models, XJ650 Turbo*, and XJ1100

NOTE: *the yamaha parts books reference the use of #165 pilot air jets for the 1982 XJ650 Turbo models, and #155 pilot air jets for the 1983 XJ650 Turbo models. Original carbs that we have explored have the #170 jets as shown above.


On Hitachi HSC-32 model carbs, there are also two air compensator jets—one on each side of the carb throat, facing the airbox—which are always a #36 size on those models which use them. The HSC-33 model Hitachi carbs, used on the XJ700 non-X models and XJ750E-II and XJ750RL models, do not use air compensator jets.

Mikuni carbs also do not use air compensator jets, but, the main air jet and, on some models, the pilot air jet are located in this position… the sides of the carb throat, facing the airbox.


Please be aware that quite a few—perhaps 60-70%of "first time" carb cleaning/rebuilding efforts end in frustration and failure, and it's mainly due to NOT doing a thorough enough job of internally cleaning the tiny, miniscule fuel passages within the carb body. Although the air passages within the carb body are (relatively) huge, and thus do not normally accumulate fuel sludge and varnish, the fuel passages-particularly the pilot fuel circuit passages—-are tiny beyond belief and even a "speck" of old varnish, grit, crud, etc. will cause problems…….in essence, that restriction becomes an "unintended" fuel metering jet, with very real consequences. This is why you want to get really, really anal in cleaning them. Many times, simply "dipping" or "boiling" the carbs out is NOT sufficient; you have to do all that AND mechanically probe (scour) these passages with tiny, fine wires to loosen such material, and THEN blast it away under pressure with carb cleaner, compressed air, etc.

And you might have to repeat that process a few times to truly get them clean.

And the fuel supply tubes and their passages inside the carb body…..which are rather large……these also get filled up with accumulated junk and debris, and thus also require mechanical intervention to loosen and remove such build-up.

The absolutely worst outcome that can occur is that your "cleaning" efforts results in loosening, but not fully removing, such stuff within the internal passages; and once you get fuel and vacuum pressures working through the carbs again, all that gunk THEN starts moving again, and re-clogging things up (fuel circuits, float needle valves, etc.).

So don't be fooled by having the VISIBLE areas of the carbs clean; although that's important, too, it's the invisible areas that matter most, and they had better be spotless, too…….or you'll probably soon be undertaking Carb Rodeo #2, after many frustrating hours spent trying to "tune" a set of carbs that no amount of tuning will ever set right!

Here are some typical problem areas that are deserving of your special attention:

- Clean the Emulsion Tubes thoroughly. This means inside and out, and every one of those tiny little holes along the bore must be completely clean, and whatever you do, DON'T ENLARGE THE SIZE OF ANY JET HOLES WHEN CLEANING THEM!

- Remove every piece of rubber diaphram or o-ring and gasket material before soaking in carb dip, as carb dip will eat them up and make a gunky mess of things.

- Clean all passages, both in the component parts and in the carb and bowl bodies, ultra-thoroughly: overnight carb dip, then ultrasonic, then wire brushes of various sizes and metal wire probes, then spray cleaners, then compressed air, then do it with spray and compressed air over and over again until you're DARNED CERTAIN THAT THE PASSAGE IS CLEAN. Then it wouldn't hurt to do it once or twice more, just to be sure.

Hint: don't overlook those fuel supply tubes (in-between the carb bodies) and their seating area and fuel supply passages within the carb bodies…..they are usually full of junk!

How will you be sure that your passages are "zestfully clean"? If compressed carb cleaner blown into a passage does not exit the other end of the passageway FREELY and FORCEFULLY, then that passage isn't clean.

- Clean the Carb Bowl Starter Jet and its pickup tube thoroughly: overnight carb dip, then ultrasonic, then wire brushes of various sizes and metal wire probes, then spray cleaners, then compressed air, then do it spray and compressed air over and over again until you're DARNED CERTAIN THAT THIS PASSAGE IS CLEAN. Then it wouldn't hurt to do it once or twice more, just to be sure. This jet, because of it's non-removable nature and it's deep, almost-inaccessible location deep within a well down in the carb bowl, is the hardest-to-clean passage in the entire carb!

And the failure to completely clean this starter jet will cause your choke to be barely or non-functional, and is the answer to the oft-asked question: "Why is my bike so hard to start?". In reality, assuming that the rest of your engine is in good condition (good battery, good compression, good coils, good plugs, and getting fuel), your bike should start up almost INSTANTLY even in sub-freezing temperatures.

So how will you know when your starter jet is really, truly, zestfully clean? One of two ways, and I recommend using BOTH:

a) shine a strong penlight or mini flashlight into the bottom of the bowl, where this jet passage "intake" is located. Look through the top of the bowl down into the jet passage "outflow" passage (this is the passage that the brass suction tube in the bottom of the carb body actually fits down into). Focus your eye carefully on the jet opening and make sure it's clean. P.S. it helps to do all this while in a darkened area….

a) put the spray tip (you may have to gently shape the end of it to a fine point) of a can of carb or brake cleaner into the intake opening of the starter jet and let rip a spray. A STEADY AND POWERFUL STREAM OF FLUID SHOULD COME OUT OF THE OUTFLOW PASSAGE ON THE TOP OF THE CARB BOWL IF THE JET IS PERFECTLY CLEAN AND OPEN. I mean this stream will absolutely spit out a good 5-10 feet. If the stream isn't powerful and laser-like precise coming out of the jet, then the jet isn't zestfully clean…..

By the way, when using the spray-stream method of checking the jet, don't even THINK of putting your eye or face anywhere even NEAR the jet outflow path, unless you'd like a painful and potentially serious trip to the emergency room.

This is what you'll want to see when you're done! (throttle-blip to Gamuru for the exceptional write-up and video!):…=8918.html

- GENTLY polish the Vacuum Piston with 1000-grit or finer sandpaper until it's smooth as a babies bottom. And always use some type of lubricating fluid while "polishing"… it WD-40, brake fluid, cutting fluid, etc. Do NOT attempt to remove any "discoloration" from the vacuum piston, these pistons are actually aluminum with a very thin brass anondized outer coating, and if you remove this coating, Very Bad Things will happen. The discoloration is normal and not harmful in any way.

- GENTLY hone and then polish the Vacuum Piston BORE in the carb body starting with 600-grit sandpaper and work your way up to 1500-grit or finer………until it's smoother than a babies bottom. And always use some type of lubricating fluid while "polishing"… it WD-40, brake fluid, cutting fluid, etc. You want the vacuum piston to be able to fall within this bore in a nearly "frictionless" manner, able to pass the infamous "CLUNK TEST" with flying colors:…=1640.html



Now, you're probably thinking that we're going a little overboard about this whole cleaning and polishing routine-and you could have a point there-but you'll feel like kicking your own butt if you go to all this work, put the carbs back together, and then discover that it still doesn't run correctly because they're still dirty/clogged inside. AND THAT'S THE MAIN REASON WHY FIRST-TIME "REBUILDERS" DO ALL THE WORK AND STILL HAVE CARB PROBLEMS——because they didn't really take the time, and make the effort, to get those carbs INTERNALLY "operating-room clean".


The carb bodies are an aluminum alloy, while the carb bowls (most of them) seem to be a more pure aluminum or magnesium. Carb cleaner "dip" and ultrasonic cleaner solutions may "stain" these metals and make it a dark grey color, perhaps even with some runny, ruddy "streaking". If you want them to look like new, here's what you do:

a) glass bead blast them. This is has the unpleasant side-effect of REALLY clogging up all the tiny passages, so an unbelievably thorough cleaning is necessary afterwards. Better to plug up all the internal passages (screw a carb hat on top, and a bowl onto the bottom, and tape/plug up as many internal access openings as possible before doing this). However, they will be REALLY bright once done, and with a velvety finish.

NOTE: do the carb dip and ultrasonic cleaning BEFORE you blast them, or they'll just turn dark grey again!

After blasting, you'll be cleaning the carbs with carb or brake cleaner and compressed air ONLY, in order to prevent staining them again.

b) metal shot tumble the bodies. In an industrial tumbler. Not a home version that you buy at Harbor Freight for $49.95. It will take 45 days of tumbling per carb body to do it that way. Find an industrial metal processing place in your town and have them tumbled with the proper size metal shot and, after about an hour or so, they'll come out looking almost like they've been chrome-plated. Price vary, locally we can get a complete set done for about $95 down here, slightly less if multiple sets are done at the same time. P.S.: by "industrial" we mean a tumbler that's about 12 feet in diameter and uses 440V 3-phase electrical service to run, and shakes the whole 10,000 square-foot warehouse room that it's in.

P.S.S.: we use custom-made passage block-off plates and rubber plugs of the appropriate sizes inserted into all "blind" screw and passage holes before tumbling, so that no metal shot can get up into such passages or holes, cause if they do, you'll never get them out, and you will hate yourself forever for allowing such a mistake to occur.

NOTE: do the carb dip and ultrasonic cleaning BEFORE you tumble them, or they'll just turn dark grey again!

- If you want to PROTECT your blasted or tumbled carbs afterwards, use some fuel-resistant clear coat. Allow for three coats with dry time in-between. So plan on a day of spray time.

- If you're going this far, you might as well replace or re-plate your chrome carb hats. And re-plate or spray "gold cad" paint on your brackets, throttle arm levers, etc. Zinc cad (silver) spray works great on upper and lower rack bars, starter circuit finger levers, etc.

- If you do all of the above your carbs will look so great upon re-assembly that you will probably want to go to an art supply store and get a shadow box and permanently mount them in there and display them on your mantle, etc. You can then buy another set of used carbs off eBay and rebuild them for daily use on your bike……….


Rule #1: never, EVER rush.

Rule #2: use anti-seize on all bolts during re-assembly, except for upper and lower rack bolts, which use semi-permanent (purple) thread locking compound.

Use anti-seize on all bolts, screws, etc. during re-assembly.

Use anti-seize on all bolts, screws, etc. during re-assembly.



Rule #3: use 100% pure Silicone Grease (and NEVER, EVER A PETROLEUM-BASED GREASE) to hold o-rings and the vacuum piston diaphram in place during re-assembly.

The Rest of the Rules:

- Do not over-torque anything.

The Vacuum Piston and Diaphram:

- When you re-install the vacuum piston Diaphram into it's bore, please note that the entire outer edge of the diaphram ha a round "bead" that sits within a machined recess on the top of the carb body. Over time, the diaphragms can shrink a small amount, and thus the edges of the diaphram will try to "pull out" of this recess while you are installing the spring and the chrome cover (or "hat"). A good way to prevent this from happening is to smear a bead of 100% silicone grease (and never any petroleum-based product, which will attack the rubber) into that groove, which will act as a "glue" and prevent the lip from withdrawing.

Also, notice that the outer edge of the diaphram has a small half-circle LOCATING TAB that fits into a matching recess on the carb body, and make sure that you align that tab with the recess.

The Idle Mixture Screw Settings:

- Installation and initial setting of the Idle Mixture Screws:

NOTE: idle mixture screws are also referred to as "pilot mixture screws" or "pilot screws".

If you've never delved into the guts of your carburetor idle mixture screw port, it can sometimes be confusing. Here's the playbook for this critical area:

If we were looking at a cut-away of the carb body/idle mixture screw bore, the order of parts, from top to bottom, would be:

^ TOP or UP ^
e) the anti-tamper cap.
d) the idle mixture screw.
c) the tension spring.
b) the HCP896 spring seat washer.
a) the HCP895 o-ring

What is mostly confusing is the order of the o-ring versus the spring seat washer….which goes in the hole first? The answer is that the HCP895 o-ring does. The HCP896 washer primarily acts as the lower "seat" for the HCP6573 tension spring, while also keeping the o-ring firmly and securely in place.

The initial setting of these screws is as follows:

  • 1980-81 XJ650 Maxim and Midnight Maxim, and XJ650RJ Seca Hitachi carbs with the coarse-threaded idle mixture screws: 1-3/4 to 2 full turns out from their fully closed (their fully seated, or "gently bottomed") position.
  • All Hitachi carbs (other than the above) which will have the fine-threaded idle mixture screws: 2-1/2 to 3 full turns out from their fully closed (their fully seated, or "gently bottomed") position.
  • All Mikuni carbs: 2-1/2 to 3 full turns out from their fully closed (their fully seated, or "gently bottomed") position.

The Floats and their Pins:

- Make sure that you re-install the Floats in their correct orientation…….believe it or not, it's easy to install them "upside down"!

And of course, as per the previous discussion on the issue with the dis-assembly of the float pins, be ultra-careful when re-installing them, as you don't want to be this far along in your rebuild process and pop off one of those float pillar arms!

The Bench-Synch Process:

The bench synch process is great for getting the bike STARTED UP and running (even if poorly) after having had the carbs apart, allowing the motor to be started and running just well enough (which may mean: barely running, at all) so that you will have the opportunity to do a proper "running" or "real synch", along with the associated twiddling, fiddling, and setting of the idle mixture screws, idle speed screw, etc.

BENCH SYNCH: Get your motor runnin'…….

REAL SYNCH: Head out on the highway, lookin' for adventure, and whatever comes our way, blah-blah-blah, etc.

- So make sure you Bench-Synch the carbs after re-assembly. This merely means that you adjust the "synch screws" on the carb throttle shaft end brackets so that you get an equal amount of butterfly valve opening (visible light passage) from each carb. This process allows the bike to start up easily after the carbs are re-installed onto the engine, and is NOT part of the further carb tuning steps that you'll need to take to actually "synch" and tune your carbs.

The amount of light or "gap" isn't absolutely critical, but what is critical is that whatever gap is achieved is fairly equal on all four carbs. Whether you use a paper clip, a matchbook cover, a business card, or whatever…….it's not the amount of the actual gap (within reason!) that's important, it's that whatever gap is achieved needs to be nearly equal on all the carbs.

But BEFORE you Bench Synch:

If you have removed the butterfly valves (for whatever reason), then while reinstalling the butterflies, hold the carb up to a light source and look through it. The butterfly valve mounting holes are slotted so that the butterfly position can be fine-tuned. Assemble with the screws very lightly tightened, then hold the carb up to the light so you are looking through the throat. Adjust the butterfly valves so they make a light-proof seal all the way around it's circumference in the throat.

If you can see light, then air will leak past at idle AND the butterfly will probably bind in the throat when things get up to operating temperature. It cannot be stressed enough that the butterfly must seat fully when allowed to close gently under spring pressure without any additional force. NOTE: if in the rack, the #3 carb must have the idle stop screw fully backed off so it does not prevent closing).

Finally, tighten the screws fully and recheck to make sure they didn't move!

NOTE: do not confuse the Idle Speed Adjustment Screw—that's the big huge one, facing to the "rear" of the bike, dead-center in-between the center two carbs—with the small "Synch Screws which control the at-rest throttle shaft rotational position, and thus the (butterfly) throttle valve opening positioning. These "synch screws" are on the throttle shaft end brackets, have a phillips-drive head, and are extremely fine-threaded little beats (the fine threading is what makes tiny adjustments possible), and only three of the four carb shaft end brackets have these screws (the #2 carb doe not have such a screw).

Float HEIGHT sets the Fuel LEVEL:

- When it comes time to Set the Float Height and thus the Fuel Level within the carb bowls, our advice is to get all the proper tools and fixtures ready, and then bribe someone who knows how to do it to come over and let them do this task for you!

It's not rocket science, but it sure can be a messy, pain-in-the-butt, trial-and-error procedure…….especially if this is going to be your first fuel-level rodeo.

If that's not possible, then read the manual and proceed accordingly. But please take the time to set the fuel levels correctly, not just by the float height measurements, but via the fuel level "sight tube gauge" method-as the proper fuel level in the bowls is a critical issue in whether the carbs perform properly or not. NOTE that the "float height" measurement is just a "shortcut" method used to try to acquire the correct "fuel level" measurement, as it is the fuel level measurement and specification that must be met in order to insure that the carbs perform properly:

  • Fuel levels that are too low results in a lean mixture.
  • Fuel levels that are too high results in a rich mixture.

In order to achieve the proper fuel level… measured by the "clear tube method" outlined in the service manuals……there obviously will be some particular "float height" that will achieve that FUEL LEVEL. But that actual float height can vary between various carbs due to a number of factors; for example, in the case of the Hitachi carbs, it also depends on which set of float valves and needles were originally installed by Hitachi (we've seen three different types in original carbs) AND which replacement style of needles and seats that you purchased. The factory specified "float height" is for an original carb, using the original float needle seats and valves, which may no longer be available for your bike.

The float height determines what the fuel level within the bowl will be (since the float height determines when the float valve needle assembly "cuts off" fuel flow from the gas tank into the carb bowl), and thus an adjustment to the float height is how you adjust the fuel level. The float height is adjusted by very small amounts of bending the float TANG ARM on the float supporting structure. This tang is the central metal "arm" that contacts and supports—via the little float needle clip - hook (on most models)—the float needle itself. You would bend this tang down (away from the needle) to raise the float height (and thus raise the fuel level in the bowl), or you would bend this tang upward (towards the needle) to lower float height (and this will lower the fuel level in the bowl).

Please note that although you typically are messing around with the float height and adjusting that tang WHILE THE CARBS ARE UPSIDE DOWN—so you can easily access the floats—the descriptive actions such as "downward" and "upward" that you may read about in various manuals are describing such directional movements in relation to the normal orientation of the carbs as on the bike! THIS CAN BE INCREDIBLY CONFUSING WHEN YOU FIRST START PERFORMING THIS ADJUSTMENT PROCESS!!!

Also note that the fuel level check (with the sight tube gauge) must be done with the carbs oriented in the same manner as if they were installed on the bike, meaning perfectly level from side-to-side (you'll want to use a bubble-glass level to make sure they are perfectly level) and completely "vertical". NOTE: the "vertical" orientation confuses some people, as the carbs, when installed on the bike, are actually tilted a bit forward from true vertical in the normal riding position. However, when checking the fuel level with the sight tube gauge, the carbs should be in both a horizontally and vertically level orientation.

Here are some other useful hints, culled from the factory service manual, regarding how to prepare yourself for the correct check of the float height measurement:

- Hold the carb in an upside down position.
- Incline the carb at 60-70-degrees so that the end of the float valve does not hang down as a result of the float weight.
- Measure the distance from the mating surface of the chamber without the gasket in place, to the top of the float.
- NOTE: the float should be just resting on, but not depressing, the spring loaded inlet needle.

Well, okay! Now, although you may not have noticed it immediately, there is a very important and very cryptic passage in the above set of instructions, which is this:

"Measure the distance from the mating surface of the chamber without the gasket in place, to the top of the float."

The "mating surface" is, of course, the bottom of the carb body (where the float bowl gasket would be located) but without the gasket in place…….check. But the next part……the "to the top of the float" bit……well, that's a bit more mysterious, because:

a) of course, as it with the "direction warning" issued earlier, the "top of" the floats, when viewed with the carbs being upside down in your hands (which is the only way to perform such measurements), is actually the physical bottom of the floats!

Told you this would get interesting…….

But that little detail aside, now we have to explore the more challenging part of that statement, which is "what and where, exactly, is the "top" of this bottom of the float?"

This simple question is, unfortunately:

a) not so easy to answer, and…..

b) varies by which carb model you are holding, upside down, in your hands and trying to measure……….

For some models, the information as to exactly WHERE to perform this measurement at the float is either not known, or incredibly confusing…..and the factory service manuals are not much of a help in this regard, what with their tiny, grainy images. It seems as though Yamaha wanted people to focus on the fuel level setting (of course, as that is the end result you're actually shooting for), and figured it wasn't worth the trouble or effort to go over the float-height-measurement procedures necessary to achieve that-—-rather, they tell you what fuel level measurement to acquire, and then tell you to just "bend that tang arm" and re-measure the fuel level and just keep going back-and-forth until you get it right! And then do that messy, smelly, spill-gas-all-over-yourself with this trial-and-error procedure three more times………

Three Blind Carbs: See How It's Done:

Because of all the considerations mentioned previously, and due to the importance of getting this job done once, and done correctly, the procedure that we suggest you follow is to work on achieving the proper FUEL LEVEL in any one carb…….so, install the needle and seat, install the float, fill the bowl with fuel, and measure the fuel level (although the factory gives a rather wide range of acceptable fuel level limits, such as 3mm +/- 1mm—in other words, anywhere from 2mm to 4mm—we feel that you should always strive to get that fuel level right in the center of the range (in the above example, right at 3mm).

You'll do this by very slight bending of that central float tang arm (the one that the needle clip fits over) until you get the measured fuel level (drain and measure a couple of times, to be sure!) that it's correct.

Now, since you know that this one carb fuel level is correct, and if you're using the same float seats and needles in all the other carbs, then simply measure the "float height" of this now known good carb…..and, you can "measure" that height in any way and at any point that you desire, preferably whatever measurement is the most consistent to perform and the easiest to see…….and then that becomes your "float height" for all of the other carbs, too. Just adjust the other float tang arms to achieve that same unique "float height", and you're done!

BUT, I would still fill and measure each individual carb and check the fuel level, just to be sure, before you install them on the bike!

The above may seem like a more time-consuming and "tedious" way of doing things, but it is the only way that we know of to make CERTAIN that your fuel levels are correct……..rather than having a "correct" float height…….and as we stated earlier, it is the fuel level that is the end result that you're seeking, not any particular float height (except for the necessary float height needed to get to that proper fuel level, of course!).

The proper FUEL LEVEL is a vastly over-looked, but vitally important part of having your carbs perform properly, as improper fuel levels WILL result in either lean or rich fuel mixtures, and will most likely tempt you to try to "compensate" for such a situation by other, NON-NECESSARY and perhaps COUNTER-PRODUCTIVE changes in other circuits or carb settings, which may affect and degrade other areas of the engine performance. IT IS ALWAYS BEST TO ADDRESS AND SOLVE THE "ROOT" PROBLEMS (or, to put it another way, to FOLLOW PROPER PROCEDURES AND ACHIEVE PROPER SETTINGS) in all of the basic areas.

This method will allow you to actually save time, money, effort, and achieve more accurate, consistent, and reliable results!

Fuel Level Cheat Sheet:

Hitachi all HSC32 series models:

-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

Hitachi HSC33 series (XJ700 models):

-Fuel level measured via the clear-tube gauge: 1.0mm +/- 1mm (.039" +/- .039")

Hitachi HSC33 series (XJ750E-II model):

-Fuel level measured via the clear-tube gauge: 5.0mm +/- 1mm (.197" +/- .039")

Hitachi HSC33 series (XJ750RL models):

-Fuel level measured via the clear-tube gauge: 1.0mm +/- 1mm (.039" +/- .039")

Mikuni BS28 (all 550 except 550L):

-Fuel level measured via the clear-tube gauge: 2mm +/- 1mm (.08 +/- .039 inches)

Mikuni BS32 (all 550L):

-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

Mikuni BS30 (all 650 Turbo):

-Fuel level measured via the clear-tube gauge: 2mm +/- 1mm (.08 +/- .04 inches)

Mikuni BS33 (all 700-X and 750-X):

-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

Mikuni BS35 (all 900RK and RL):

-Fuel level measured via the clear-tube gauge: 5mm +/- 1mm (.20 +/- .04 inches)

Mikuni BS36 (all 900F, FN, N, etc):

-Fuel level measured via the clear-tube gauge: 5mm +/- 1mm (.20 +/- .04 inches)

Mikuni BS34 (all 1100):
-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

Okay, the fuel level measurement is done via the clear-tube gauge method, and this tube should be positioned along the SIDE of the carb body being measured (not along the front or the back of the carb body), and along the centerline of the carb body, basically in-line to where the main fuel jet would be located. The fuel level is most critical to the performance of the main fuel jet, so that is where the fuel level in the bowl should be measured at.

The actual fuel level measurement (which reads something like "3mm +/- 1mm") refers to the level that the fuel in the tube is located below the bottom edge of the carb body where the carb bowl is bolted to the carb body. NOTE: as fluids like to do, the "top" of the fluid level in the tube will not form a perfectly straight "top" in the tube……instead, it will be somewhat "u-shaped". The proper measuring point is the lowest level of the fluid in the tube.

Whew! I told this is a tedious process; luckily, once you get the float heights (and thus the fuel levels) set correctly, it doesn't change very much over time, so you don't have to go through this procedure very often, or maybe even ever again!

Unless………..something occurs that physically bends that float tang arm, and thus changes the float height and thus the fuel level in the bowl. Besides "time" and "metal fatigue", the main culprit that can bend this tang arm is excessive pressure on this arm, typically caused by a stuck, seized, or jammed float needle, which holds the needle open and lets fuel keep coming into the bowl….thus the fuel level rises, pushing the float upwards, and the tang arm exerts excess pressure against the float needle, and ends up bending the tang arm due to such excess pressure.

Float needles can become stuck due to defects in the needle itself, the cylinder bore of the needle seat, or perhaps a piece of gunk from your fuel tank made it through the filters and gets lodged into the needle seat, seizing the needle open or preventing the needle from ever closing fully against the seat. Then fuel keeps flowing into the bowl, the float level rises, excess force is applied to the tang arm, etc. A worn needle tip…..which may also prevent full closure of the seat opening…..can also cause high fuel levels.

Also, please note that on models on which the float needle has a spring-loaded "tip", that this tip should NOT depressed when setting the floats, nor should it be seized open nor closed. That needle tip is a "shock absorber" and does not and should not be depressed—at all—when setting the float heights. If that spring-loaded tip becomes seized shut, then the floats will have to be set very "high" before they ever rise far enough to exert enough pressure on the collapsed tip to force the needle closed, and thus will cause bowl flooding.

So: if your carbs flood, then the tang arm can bend, and even if you solve the flooding problem, you may find that after such an event you may have a continual fuel level problem.

Moral of the stories: check your fuel levels periodically (you can do this with the carbs still on the bike) and adjust them into specification if they are not!

Okay, if you've read all of the above and are not fully confused, well, it means that you just haven't been paying enough attention……….


NOTE: upper & lower rack screws and butterfly-to-throttle shaft retaining screws use semi-permanent thread-locking fluid; all other fasteners use anti-seize.


a) make sure anti-seize compound doesn't get into a passage where it will undo all your careful attention to cleanliness, and….

b) disregard the torque specifications of the fasteners. I draw your attention to Yamaha's standard passage regarding torque specs (included in every Yamaha service manual):

"Unless otherwise specified, torque specifications call for clean, dry threads. Components should be at room temperature."

If the specification is for lubed threads (and anti-seize grease is a "lubricant") on a particular bolt, then the Yamaha torque chart will have a little oil can symbol beside it.

c) therefore, applying full (dry spec) torque to a lubed bolt may well strip your expensive aluminum casting or brass jet. As a rule of thumb, reduce the specified torque by at least 10% if you have applied anti-seize compound to the threads of the bolt.

Hat Screws:
none given, and not much is needed! Become familiar with the concept of the word "snug".

Plastic slide piston needle retaining nut:
none given, use "snug".

Enrichment (starter) circuit brass fitting:
40 ft-lbs (???—-that's what the manual states, but it sure seems excessive!)

Float seat:
22 ft-lbs (???—-again, that's what the manual states, but it sure seems excessive!)


Pilot fuel jet: 3.5 - 4.5 ft-lbs.

Main fuel jet: 3.5 - 4.5 ft-lbs.

Air compensator jets: 4.5 ft-lbs

Pilot air jet: 4.5 ft-lbs

Main air jet: 4.5 ft-lbs

Air jet cover plate mtg: none given.

Upper rack mounting screws: 3.5 - 4.5 ft-lbs.

Lower rack mounting screws: 4.5 ft-lbs

Bowl-to-carb mounting screws: 3.5 - 4.5 ft-lbs.

Bowl fuel drain screw: 4.5 ft-lbs

Throttle shaft nuts: none given.

Fuel Tank and Petcock Torque Values, Just For Fun:

Petcock mounting screws to tank: 4.5 ft-lbs

Gas cap mounting screw to tank: 3.5 - 4.5 ft-lbs.

Sender unit to tank: 4.5 ft-lbs

Tank emblem screws: none given

The Book of Job: QUALITY IS ALWAYS JOB #1:


  • time.
  • patience.
  • paper, pen, and a notebook.
  • a clean, semi-uncluttered, well-lit and well-ventilated area to work in.
  • a strong flashlight.
  • a magnifying glass.
  • a digital-cam would be a huge bonus.
  • haynes or clymer workshop manual.
  • yamaha service manual.
  • threads already bookmarked.
  • paper towels and shop rags.
  • access to ultrasonic cleaner.
  • access to tumbler or glass-bead blaster.
  • Flat & phillips head screwdrivers, various sizes. JIS phillips-drive screwdrivers should be used.
  • wire tube brushes, various sizes, for cleaning internal passages.
  • brass & steel wire toothbrushes, for cleaning the outside surfaces.
  • mini, mini-plus, or man-sized passage cleaning wires set.
  • Engraving pen or punch to mark carb bodies, bowls, throttle shafts, etc. Ink won't do, even "sharpie" style permanent ink markers, since carb dip and spray cleaners will remove them.
  • Hammer.
  • METRIC open-end wrench and socket assortment
  • pry bar or soft mallet.
  • channel locks, vise grips, pliers—-needlenose and otherwise.
  • dental picks.
  • mini drill bit for carb bowl starter jet cleaning.
  • e-z outs and/or LH drill bits.
  • small pencil torch or a heat gun.
  • vise.
  • drill & drill bits.
  • float height ruler or gauge.
  • fuel level sight tube.
  • a 6" or 12" bubble level.
  • a carb rack Alignment Plate, necessary as you re-assemble the individual carbs together and tighten the upper and lower rack bar screws. The four carbs must be placed on a perfectly flat surface as the screws are tightened or mis-alignment will occur, and life gets really un-fun if that occurs………
  • 100% silicone grease. Any petroleum-based grease will attack rubber.
  • anti-seize fluid.
  • semi-permanent (purple) thread locker fluid.
  • brake or carb cleaner (and lots of it).
  • carb dip or access to it.
  • Fine sandpaper, automotive paint grade, 400-, 600-, 800-, 1000+ grit.
  • restoration paints and coatings, if you are so inclined to do such detailing.
  • synch sticks.
  • YICS block-off tool, if your bike is has a YICS passage.
  • YICS port passage cleaning tool, if your bike is has a YICS passage.
  • gun brushes or similar.
  • gun cleaner or similar.
  • colortune plug.


Carb rebuild:

  • throttle shaft seals (if you split the carbs from the rack).
  • fuel supply tube o-rings (if you split the carbs from the rack).
  • float needle seat rebuild kit (seat, needle, clip, seat washer).
  • bowl gaskets.
  • idle mixture washers & o-rings, screws if damaged.
  • jets, if damaged or incorrectly sized.
  • carb bowl fuel drain screws.
  • new replacement carb hardware (screws, etc.)—-now is the time to upgrade to stainless steel and/or allen-head fasteners if so desired!

Valve shim replacement:

  • shims as needed.
  • shim bucket tool.
  • feeler gauges, .002" - .012" (inches, not metric!)
  • 19mm open end wrench.
  • valve cover gasket and end caps seals as needed.
  • valvecover bolt pressure washers.
  • glue for valve cover gasket.
  • valve cover gasket.
  • crankcase oil pump cover gasket.
  • small flat-bladed screwdriver or pointed pick tool.
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