The Secret Life Of Carburetors



Or, why carbs suck:

At first glance, you might read the title of this article and say "so what?" You're not really interested in why carbs operate as they do; you just want them to perform properly.

But wait, grasshopper: in order for you to appreciate both the what and the why of proper carb rebuilding techniques, then you need to also have a basic understanding of what a carb is trying to do, so you'll have a clearer understanding of why you need to pay close attention to what might seem like the incredibly picky details and procedures that are recommended as part of a proper carb rebuilding process…….which you can read all about here:…14692.html

But before you go there, please continue reading here……

I'm Already Denying It; I Should Have Been A Politician:

We started out by saying that "carbs suck". Well, as we all know, that's not really true. It's the engine that sucks, and it's that "vacuum signal" from the engine—that "suck"—that controls and tells the carb "when" and "how much" they're needed to do in order to correctly perform their dirty, fuel-and-airish deeds.

And if your carbs are not ready to properly respond, or if they tend to respond in a non-appropriate or non-reliable or non-coordinated manner to that sucky signal, well, that's the root of all evil, now isn't it?

Nature Abhors One: All About A Vacuum:

Actually, there's no such thing as a "vacuum" in nature (no, not even in the new lightweight 8-pound Oreck model that you can try at home for FREE! for 30-days……..), but there are areas of higher or lower fluid pressure………"higher" or "lower" being meaningful only in comparison to some other nearby or related area……..and what everyone likes to refer to as a "vacuum" really means "an area of lower pressure".

Areas of low pressure—just like you see and hear about on TV when the weather-person is talking about weather systems (and a hurricane, as you may know, is the "mother of all low-pressure areas")will "draw in" a fluid-and physically-speaking, air is a type of "fluid"——from the surrounding areas of "higher pressure". Note that we haven't said anything at all about HOW MUCH pressure we're talking about…….nothing about p.s.i . or inches-of-vacuum or the like…….no, all that matters is that one area is at a lower relative pressure to some other area.

Now, in an engine, the monster that is responsible for the creation of a "lower pressure area" is the piston and it's cylinder. As the piston is mechanically (via the crankshaft and the connecting rod) drawn down into the bowels of the cylinder, the ring-tight seal of the piston to the cylinder walls evacuates the area of the cylinder above it. Well, even the word "evacuate" isn't good, so let's try this: when a piston is at the very top of it's stroke, and before the piston moves down into the cylinder, there was, let's say, 3cc's of total area between the top of the piston dome and the cylinder head combustion chamber. And, there was 3cc's total volume of air in that space, too.

Good. Now let's say the piston moved all the way down to the bottom of that cylinder. Now, there's maybe 120cc's of total area in that cylinder (between the top of the piston dome and the cylinder head combustion chamber), but still only 3cc's of air volume to fill this newly expanded area! That air thus becomes "thinned", in essence, and now we ram head-on into one of the basic laws of physics:

A fluid (and once again, air is a "fluid" in physical terms) will undergo a decrease in pressure as it expands or is "thinned".

Hey, I didn't make up the laws of physics, I'm just reporting them…….

So the downward movement of the piston within the cylinder "stretches" and "thins" the amount of air that's in the cylinder. And this "thinned" air loses pressure.

Bingo! A "low pressure area" has been created. Your piston, in a conspiritorial bond with both it's rings, the cylinder walls, and the cylinder head, in a plot worthy of a Tom Clancey novel, has just created a "vacuum", better thought of as an "area of low pressure".

An Army of Air On The March and Spoiling for a Low-Pressure Fight:

Now, in the presence of this developing low pressure area within the cylinder, what happens? Well, my-oh-my, an intake valve opens! And that intake valve opens up a sealed route through the intake passage in the cylinder head, and then into the intake manifolds, and which continues on into the carb throat (and beyond). And guess what? This entire air supply in the cylinder head passage, and in the intake manifold, and thus in the carb throat (and beyond, all that way back to your airbox boots, the air box itself, and finally to the air outside the airbox, too) "feels" the presence of this low pressure area inside the cylinder, and like the greedy beast that nature is…..abhorring such vacuums with a passion and a universal vengance……seeks to invade this low pressure area and "fill it up" until the low pressure area is no more, and the air pressure throughout this air supply chain is equalized.

So, an air stream—an invisible little river of air—within the atmosphere, thru the air box, the airbox boots, thru the carb throat, the intake manifolds, the cylinder head passage, and on into the cylinder, is created. An airflow. A flow of air.

And in case you weren't fully pumped yet, well hang on, 'cause this is where things really get moving…….literally!

Here's Where the Plot Thins:

This moving column of air, created by the opened intake valve that exposes that low-pressure area inside the cylinder, makes that afore-mentioned Law of Physics—just like a restored XJ-series motorcycle—fully growl to life again, and actually do something useful. Recall:

A fluid will undergo a decrease in pressure as it expands……

and because we can re-write that Law above to read as:

A moving column of air, which is the same as an expanding volume of air, will undergo a decrease in pressure as it moves (and, you actually get even more bang for your buck: the faster that air moves, the less pressure it exerts).

A decrease in air pressure. Wow! It's just what the doctored ordered!! Inside the carb throat, that is………..

Why? Because we've got this big puddle of fuel sitting there in the bottom of the carb float bowl, all dressed up with no way to go where we want it to go (inside the cylinder).

But thank goodness for that Law of Physics, or should I say Fluid Dynamics? No matter, it's all the same. The piston moves down in the cylinder, a low pressure area is created inside the cylinder, the intake valve opens, that air column through the head passage - intake manifold - carb throat - etc. starts it's mad (and, basically, instantaneous) rush into the relatively-empty cylinder, and this rapidly marching column of air now also exerts or creates a low-pressure area all it's own……and especially, within the throat of the carb body, where two very, very important things happen:

a) the fuel circuits….both the pilot fuel circuit and the main fuel circuit……"see" or "feel" a tug towards this low-pressure area within the carb throat, and………

b) the "vacuum piston", firmly attached to it's rubber diaphram, begins to rise. Why does it rise? Because below the rubber diaphram, there exists a relatively high-pressure area (the area below the diaphram is exposed to outside-atmospheric-air pressure) while the top of the rubber diaphram is exposed to this now low-pressure air in the carb throat…….that's the purpose of those two little holes in the bottom of the brass piston, they allow the air pressure that exists in the carb throat to also migrate up through the piston, and make their low-pressure-ness known in the area above the rubber diaphram.

Thus this low-pressure air in the carb throat is also the same low-pressure air above the rubber piston diaphram, while the underside of the diaphram is still seeing "atmospheric" (higher) air pressure. The result: the higher air pressure below the diaphram "pushes" the diaphram, and its attached piston, upwards.

And since the main fuel jet regulating needle is attached to this piston, well, there's now more fuel available to flow through the main fuel jet, since that restrictive needle has been lifted up out of the way…….

"But wait", I hear you cry…….. "How does the fuel get up out of the float bowl and into the cylinder?"

Ohh-h-h-h-h, Grasshoppa'…….

And Here's How Carbs Suck:

Remember the last time you got a soft drink at the fast-food burger shack? Did you get a straw with it? And poke that straw through the little plastic lid, into your delicious sugar-water fluid, and take a long draw on it?

I'll bet if you did, you were rewarded with a transfer of fluid from the fluid bowl…..that fast-foot equivalent of a sippy-cup, or, of a carb float bowl…..into your own biological "carb throat". And this happened why?

Because your "suction" on the straw evacuated the air from the straw, and thus created a low-pressure area in the straw (above the fluid that was in the straw), while the fluid in the bowl (cup) still experienced normal, atmospheric (14.7 p.s.i., if you insist on dealing in absolute numbers) air pressure………which of course, is HIGHER than the lower pressure area you just created in the straw.

Therefore, the fluid (fuel) in the cup (bowl) now feels a positive pressure acting upon it, and just as fluids of any type are wont to do, they move from areas of higher pressure (in the bowl) to areas of lower pressure (up the straw, or up through the fuel circuits in the carb body, all of which end up with their other opening in the carb throat).

Boom! Just like that, we now have one fluid movement (air through the throat of the carb, created by the downward movement of the piston in the cylinder) daisy-chaining another movement of fluid (gasoline up out of the carb bowl and into the throat of the carb). And it's all caused by your old friend, the vacuum, now d/b/a as a low pressure area.

A Brief Musical Interlude:

Now, in-between the fuel in the bowl of the carb, and the starter circuit, pilot fuel circuit, or main fuel circuit exit points in the carb throat, are a variety of regulatory valves or restriction points. We call these "jets", and in the case of the choke circuit, we may want to completely turn such a circuit on or off, in which case we also have the starter plunger valve.

In the case of the pilot (idle) mixture circuit, we also may want to be able to "fine tune" the amount of restriction in this circuit, so we have an "idle mixture screw" that allows us to make a static (fixed) additional amount of more-or-less restriction to this circuit, depending on the other operating parameters of the engine.

And finally, in the case of the main fuel circuit, we have a movable main jet "needle" that moves up and down within the "power valve" (sometimes called an emulsion tube or the main needle jet) and thus are able to create a variable amount of more-or-less restriction to this circuit, depending on the other operating parameters of the engine.

And here's the really neat part of the whole system, and the "genius" part of these otherwise "simple" carbs: on this main fuel circuit, besides the "static" (fixed) circuit restrictions caused by the main fuel jet and the emulsion tube, there's also that "variable" amount of restriction within that emulsion tube caused by the rise or fall of the main jet needle, and that variation is ALSO caused by the same reduced pressure in the carb throat, due to the variable speed of the airflow through the carb throat (and thus the variable amount of pressure drop due to that varying speed of the air column).

And do you recall that neat little party trick from earlier? How the low pressure in the carb throat causes the area above the rubber diaphram of the "vacuum piston" to also become a low pressure area, and thus the somewhat constant high-pressure area under that "big top circus tent" of a diaphram then "pushes up" on the vacuum piston unit, and the main jet needles moves up and down with it? And as the air pressure in the carb throat varies (in response to engine rpm——that "moving column of air" either moves faster at higher rpms, or slower at low rpms, and thus creates a greater or lesser area of "low pressure" in the carb throat) then this varying amount of low-pressure above the diaphram either really pulls on that heavy vacuum piston (and needle!) strongly (and thus higher), or, when the vacuum signal decreases in strength, allows the piston to fall back down in its bore, and the needle more fully fills up the inside of the emulsion tube……and thus lessens the amount of fuel being sucked up though the main fuel circuit.

Thus our variable amount of low-pressure within the carb throat not only sucks the fuel up out of the bowl, and through the various restricted (the role of the fuel jets) passages, but that same varying vacuum signal also allows us to further regulate the amount of fuel flow, at various engine speeds and loads, thru the main fuel jet system.

Now tell me that's not clever…………

The Zen of Hitachi or Mikuni Carb Rebuilding:

Okay, now we get to wrap up all the loose ends and you'll more fully understand all of the reasons why we suggest that you should do or really oughta do or absolutely must do all the tasks that we blabber on and on and on about during the carb rebuild procedures that are explained more fully elsewhere.

-1- your job as a "carburetor extraordinaire" depends upon your ability to precisely meter just the right amount of fuel into the carb throat, at precisely the right time.

-2- in order to do this precisely and reliably, your fuel passages must be properly regulated (the size of the jet restrictions) and only have those restrictions (no partially plugged jets, or gunk in the internal passages to change that amount of regulation).

-3- the variability of the main fuel circuit delivery depends on the vacuum piston moving in precise coordination with the changing vacuum signal (low pressure conditions) in the carb throat.

-4- everything depends on that vacuum signal-the low pressure area—created within the cylinder by the downward movement of the piston.


a) if your valve shim clearances are not in specification, guess what happens to that vacuum signal (low pressure area) within the cylinder? It's weaker than what it should be, because a valve that is open too much, or too little, relative to the timing of that downward piston movement, does not allow a sufficient or proper low-pressure area to be created.

And what's more, if some of the valve clearances are correct, and others aren't, then guess what? EACH CYLINDER IS PRODUCING A DIFFERENT VACUUM SIGNAL TO ITS OWN CARB, and thus each carb is "pulling" different amounts of fuel out of its carb bowl, and therefore delivering different amounts of air-fuel mixture to that cylinder. Thus each cylinder is producing a different amount of power.

And if you recall (or will discover, if you have not already learned) that the entire "engine synch" process is really about "tuning down" (reducing the output of) each cylinder down to the output of the weakest cylinder! So weaker vacuum signals in a particular cylinder—caused by valve shim clearance issues on a particular cylinder—-causes you to further decrease the entire engine's output during the synch process as you "down tune" the good cylinders to the performance of the worst cylinder.

b) and if all of the above wasn't bad enough, you've also got that reduced vacuum signal (differing amounts of low pressure in each individual cylinder) acting upon the vacuum piston's diaphram, and thus the weaker cylinders will also have a weaker amount of vacuum piston "lift" due to that weaker vacuum signal……and less fuel flowing through that particular carb's main fuel circuit.

c) and this is why you have to very carefully inspect the vacuum piston diaphram for any pinholes in it……those pinholes "bleed" vacuum from one side of the diaphram to the other, thus equalizing the air pressure on both sides of the diaphram, which is exactly what you don't want to have happen!

d) this is also why you want to make darned sure that the bore in the carb body that the vacuum piston moves within is as "friction-less" as possible……..we want as little (ideally, "none", but this is not yet a perfect world) restriction to the movement of this vacuum piston in response to whatever vacuum signal is present…'s the strength of the vacuum signal that we want the piston (and thus the main jet needle) to respond to, and not to be sullied by any mechanical frictional restriction of the carb bore against the vacuum piston. And we especially don't want one carb's piston to be reacting less strongly to a given amount of vacuum signal versus how the vacuum piston in the next carb over is reacting to the same signal!

To say that the above situations are some of what makes tuning and coordinating the actions of four different carbs in these set-ups a "challenging" activity would be a gross understatement.

Now you understand why we're so particular about these issues in our carb rebuilding procedures. We want to help you avoid any additional variables that the lack of a proper cleaning, polishing, or proper rebuild procedures might cause or introduce into this system…..because it's as complex as it needs to be already!

One Last Myth, One Last Epiphany:

To drive all of this home, consider this:

Carburetors used to be known technically as an "atomizer", because some advertising-type (who knew nothing about science, perhaps) felt that the end-result of the ULTRA-PRECISE fuel metering and regulation actions of a carb would end up with the fuel being "atomized" down into an ultra-fine mist of fuel particles that is necessary for extracting the maximum power from fuel during the combustion process.

Of course, no such breakdown of the fuel into atom-sized particles or droplets occurs, but at least it was a nice picture that was being painted, good enough for the lay-person to conceptualize and understand.

But, let's run some quick numbers just so you can get a real feel and appreciation for just how precise a device your carbs need to be in order to perform their special magic.

Although some of these numbers have been chosen for ease of use, they're "about" correct and typical.

Now follow this bouncing ball:

1) Let's say you're puttering down the freeway on your XJ650 at 60 mph.

2) To accomplish this, you're traveling in 5th gear, and so your engine is turning at a pretty constant 5,000 rpms.

3) And let's say that your bike gets 30 miles-per-gallon at this speed, in 5th gear.

4) So after 60 minutes, you have traveled 60 miles (you're doing 60 mph, after all), and thus your engine has turned over a total of 300,000 revolutions………..5,000 revolutions-per-minute x 60 minutes = 300,000 total engine revolutions.

5) And since your engine is achieving 30 miles-per-gallon, and you've traveled a total of 60 miles during this hour, that means you've used a total of 2 gallons of gas.

And this two gallons of gas that it took to get you 60 miles down the road and 300,000 engine revolutions is pretty much equally distributed between each of your four carbs. So each carb consumed a total of 1/2-gallon of fuel, and 1/2-gallon of fuel is equal to 64 ounces of fuel.

Per carb. To go sixty miles. At 30 miles-per-gallon. At 5,000 rpms. In one hour's time.

6) Now, since a four-stroke engine incurs a fuel-intake charge of fuel-and-air on every other revolution, then there were only 150,000 fuel intake events, per cylinder, during this journey.

And each cylinder consumed a total of 1/2-gallon (64 ounces) of fuel during this jaunt.

7) That means that we can calculate what each carb, supplied with those 64 total ounces of fuel divided by 150,000 intake-charge events, supplied to its cylinder as a per-intake-charge amount of fuel.

8a) And although that's still not at the atomic level, that works out to .0004 ounce per intake charge. That's 4/10,000th of an ounce per intake charge.

Four Ten-Thousandths Of An Ounce per intake charge is the precise amount of fuel that each carb has to regulate, meter, and deliver.

8b) Just something for you to think about………….if and when you are tempted to take shortcuts!

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

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