Inside A Motorcycle Carburetor - Float Bowl Height | MC Garage

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.

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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.

[billmackey]

Near the end you mentioned bending the float adjustment tab a "slight" bit. Wish I'd heard this in 1973. I bought a nearly new 1968 Kawi 500, not running that had gone through 3 owners while not running. I knew very little about trouble shooting, but learned a bit. (As it turned out, it had a bad distributor rotor)
But, of course, I rebuilt the carbs first thing. And the float spec in the Shop Manual was way off what my carbs had, so I bent the tab a mile, expecting that to fix the bike, but it didn't. When I finally realized that I got a hot spark from the coil, but a very weak one at the plugs, I bought a new rotor ($12) instead of a cap ($13), put it in and YES! - hot spark! But it still didn't run. But when I squirted gas into the carbs, ran fine.
FINALLY figured out that the float spec in the manual was wrong. Fixed it and had a very sweet running, screaming hot rod bike, until I loaned to a 'friend'
Another lesson learned.

[uofa]

Great information! The one thing that might help is to show the proper way to measure the float height. I think this may help because of the accuracy needed to get the fuel level correct. Thanks guys and keep up the good work.

[ralphwaters]

I've had a number of problems with "sticking" needle valves over the years, both in cars and motorcycles. The single most common issue involving adjustment (not dirt or water) is IMHO a factory spec that opens the float valve a bit too far. The problem usually occurs when a vehicle sits long enough for the float bowl to dry out from evaporation. The float drops to its full-down position, and the tab that pushes on the needle ends up at an angle far from the ideal 90-degrees. You can see this in the video around 1:08. The lateral thrust is often enough to jam the needle at an angle in its bore and flood the engine. Polishing the needle and seat bore is not enough to fix this. (Yes, I tried that too.) It's usually impractical to bend the float tab enough to tune this angle without destroying the fuel level adjustment in the process. However, the next best thing is to reduce the float travel limits from the factory spec a bit so that it doesn't drop down quite as far before hitting the stop. This will help keep the float tab pushing straight on the needle and avoid the flooding problem. Yes, you need to be sure the needle is opening enough for fuel demands at WOT, but it's usually quite a bit less than the factory spec calls out. This simple adjustment has fixed the "stuck needle" problem every time for me.

[riveraluciano]

Maybe I'm going a bit over myself here, but here goes:


Can you make a diagnosis tutorial? Basically how to quickly diagnose when something is off on the motorcycle (brakes, carb/injector, suspension, maybe basic electric system). I'm not talking about pre-ride check of integrity of parts, but rather their functioning on a daily.

I'm not necessarily asking for myself, but I do wonder just how easier would my life had been if I could have recognized what's the issue when my front brake "pumps", when I lose power while accelerating or when I hear the chain rattling after downshifting or letting off the throttle.

[billmackey]

The concept of a carburetor float and valve is very similar to the float and water inlet in a toilet tank.

[CajunGreenMan]

Nice! Never heard of the 45 degree angle thing. That will come in handy, thanks!

[sibiar]

Great video. Some drawings or diagrams could be helpful in visualizing what's going on here.

[ChopperFisher]

Outstanding video, production value is admirable!

[dmney]

I cant wait for jetting because fuck me its tedious and difficult.Great video guys super helpful

[davidhein]

thanks for the info, it will definitely be of use. Looking forward to upcoming video.

[GETUPANDGO]

I was always taught to hate carbs...but the more I've gotten into working on my own bikes, the more I like them

[mmohdx]

Good info... Next subject on the sprocket ratio.

[sennest]

In agreement with all, excellent series and an awesome episode! Thank you!

[davidbrierly]

This is an eye opener. Love it . Just rebuilt my carb and will fine tune if i have to using these new info MC inFo's ;)

[DavidReuschJr]

great videos guys! And that framed peg board art at the end... so beautiful! lol

[Shkvarka]

Hi Justin! Thanks for the nice tutorial! Could you please make a video about ignition coils and CDI boxes? How they work together and how to troubleshoot them?

[dbwindhorst]

Just learned that hard way that the float can stick on a brand new Mikuni TM40, too. Apparently this isn't unheard of, and may be traced to residue from the oil used to coat the valve.

[chris-dncl]

I want moorree videos more knowledge i love the mc garage tips keep doing videos thank you guys

[mrikomediana]

Please make a video on how to read spark plug colors. Thanks!

[TheFLUFFless]

I have been searching for the first video in this series. you haven't linked them very well. why aren't they a playlist? or at least link them in the description numbered! I watch a video to tell me if you haven't watched another video to go and watch that. What's the first video called?