Inside A Motorcycle Carburetor - Float Bowl Height

Dissecting the innards of a carburetor, beginning with the float bowl

There are a number of things that must be adjusted properly on your carburetor in order for your motorcycle to run properly. And each of those things works in conjunction with each other, but in order for a proper air and fuel mixture, you must have fuel. Today on MC Garage we talk about the carburetor float.

Last week we ran through all the basics of a carburetor, touching quickly on what everything does. If you haven't watched that video yet, stop right here and jump back to that vid. It will help with the entire picture of what is where.

The carburetor in the simplest of terms has just one function: Mixing air and fuel in the proper ratio. And to do that you need fuel. Fuel is delivered to all the carburetor’s circuits via the float bowl. It’s a pretty simple system, but if it’s not right, it can mess with everything.

How it works is fuel enters the float bowl via the fuel inlet fitting. From there it flows through a needle valve. That valve is then actuated by the float itself. When the level is insufficient, like when you are using fuel or the bowl is empty, the float hangs down and opens the valve. When the level is reached at full, the valve closes. Super-simple system, but there are some things that can go wrong.

First is the issue of a stuck needle valve. Sometimes, the needle can get stuck, whether that is a piece of crud holding it open or it’s not sliding smoothly. When this happens fuel will continue to flow and overfill the bowl. When this happens the fuel will flow out of the overflow tube. A quick fix is to tap on the side of the bowl with something like a screwdriver handle to shake the crud loose. If that doesn’t remedy the situation, the carb is going to have to come apart. Which you should do anyway if the needle is sticking.

The next issue is the needle might be worn out, also leading to overflowing or incorrect metering. When you pull the needle out, the sealing surface should be smooth. Run your fingernail down the needle; if you can feel a ridge, it’s toast. Replace the seat at the same time; the needles usually come in a seat and only run about $15 to $20 per body.

Once you know the needle valve is good, you need to make sure the float is good. First thing, make sure the float, well, floats! Do this in gasoline, as it has a different specific gravity than water or some other fluid. After that make sure it moves freely and doesn’t bind up. Once those checks are complete. It’s time to check the level.

To check the level, you will need the proper spec from your manual. This measurement will be the point at which the float just closes the needle valve. You can use a clear external tube attached to the overflow that will show the level but that is a pain. You’ve already got the bowl off, might as well measure it manually.

You want to measure the height just as the float touches the needle. The easiest way to do this is to hold the carb at a 45-degree angle. Then watch the small metal tab on the float as you move the float up toward the body. Just as the float touches, that’s your level. If you hold it straight up and down, you will have an incorrect height. The float will impart too much pressure on the small spring under the pin in the needle. That is the biggest mistake people make when measuring float height. If you need to adjust the height, up or down, just slightly bend the tab that contacts the need in the correct direction.

That's it. Once you have the float height correct you can move on to the next step, the idle circuit. Which we will cover next time on MC Garage.