The Carbs

Most problems on these old bikes can be traced to the carbs!

What are Carburetors?

Wikipedia says that a carburetor (American and Canadian spelling), carburator, carburettor, or carburetter (Commonwealth spelling) is a device that blends air and fuel for an internal combustion engine.

Think of it as the meter deciding what the fuel-to-air ratio is that's going in for the explosion. When you run out of gas and have only air going through, the engine dies. When you flood a motorcycle and try starting it while holding down the throttle and the choke, you give it too much gas and kill it. To produce the explosion necessary for making your pistons going up and down, a happy medium is needed (somewhere between 5:1 and 14.7:1 air-to-fuel, if you were wondering, with the latter being optimal).


The basic principle of operation is that when a piston goes down, it creates a vacuum. Another way to say what's going on is that the piston, going down, causes 'suction'. The only place it can pull from is through the intake valve, through the manifold boots, through the carb throat, through the airbox boots, through the air filter. Because of the design of the carburetor, however, as air is pulled along this passage way, it is sped up in the process by a venturi at the mouth of the carb. Some of the suction pulls gas from the bowl sitting at the bottom of the carb, where gas collects from the gas tank, which rides along with the air into the piston.

When the piston is all the way down, the intake valve closes, creating a sealed environment. At this point, there's nowhere to go but up. This causes a huge compression of air (9.2:1), and when air compresses, it heats up! Just when the piston reaches top-dead-center, this hot, pressurized mix of air and gas is ignited by an arc of electricity from the spark plug's tip to its body. The explosion shoots the piston down into its cylinder… But now we're not talking about carbs anymore!

Carbs are, as we said, what makes that ideal mixture happen. But to make this happen, there have to be passages that are so small that most sewing needles won't fit in them. The amount of gas per explosion is tiny, a fine mist. Unfortunately, gas has a tendency to leave behind a thin film of residue if it's not constantly moving. And when we put these two together, we get the doubly inconvenient problem of a film that changes the amount of gas that can flow through the passages—often eventually blocking off the smaller passage conveniently. (Causing huge problems!)

Given that these bikes are 30 years old, and that most people are terrified of working on their carbs, most of these bikes are suffering from carb problems and in dire need of service!

There are 3 types of carbs used among the XJ bikes:

  • Mikuni's for the smaller ones (550's and such)
  • Hitachi HSC32's for the mid-range.
  • Hitachi HSC33 for the bigger bikes.

What's the difference? Well, the Mikuni carbs are technically somewhat higher performance, and more tunable. They are also more prone to breakage and require more care when rebuilding. The HSC32 and HSC33 are almost identical, but have some moderate difference.

A dissection and exploration of some of the carb's inner parts can be found here.

Wanna clean your carbs? Great, there's a lot to know, so read up! It looks scary, but once you've had them apart once, you realize they aren't so bad.

(Before you get into your carbs, though, you have to do your valve adjustments. You did do that, right?!)

The Whole 9 Yards: Rick's Old School Carb Cleaning Guide

The Secret Life of Carburetors

Church of Clean

Pictures of Hitachi HSC32's

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