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Yamaha Dirt Bike Carb Leaks Fuel from Overflow TubeIf you have the problem of a Yamaha dirt bike carb leaks fuel from overflow tube, then there are a few possible issues that might be causing this. First of all, has the carburetor been disassembled just prior to the problem appearing? If yes, then it is possible that the float assembly was messed up when it was taken apart, and now the float inlet needle valve can not shut off the inflow of fuel. As a result, the fuel level in the carb will overflow. If this is the case, then you will need to remove the carb from the bike and take off the float bowl and check the float assembly and float height setting. This scenario is probably not the most common case , because most people will not have just taken apart their carburetor. More likely, the bike just sat for a while and the problem was noticed when the bike was taken out to use again.
Another possibility as to why your Yamaha dirt bike carb leaks fuel from overflow tube is that a small piece of debris worked its way from your fuel tank into the carburetor. This small foreign object can lodge between the inlet needle and the valve seat. This can cause the float valve valve to remain stuck open allowing fuel to overfill the float bowl and overflow. This should not happen if you have a good fuel filter. If for some reason you don't have a fuel filter installed, then you might want to pull the carb from the bike, remove the float bowl, and carefully remove the float assembly and inlet needle valve. Look for any signs of junk that might have been causing the needle valve to not be able to close. You can use compressed air to try to blow out any junk you find. The writer of this article once had a brand new motorcycle engine that had a problem like this. Fuel tank was filled and all of a sudden fuel leaking on the floor overflowing from carb! What?!?! This is a brand new machine! Carburetor was taken a part and checked for debris. Nothing obvious was found, but after things were cleaned out and reassembled the leaking stopped. This also happened on a brand new lawn tractor engine. So, junk can cause the inlet needle valve to stick open even on a new machine. However, the more likely culprit is probably going to be described below. As gasoline has been reformulated for better emissions characteristics, it has commonly resulted in fuel that has much worse storage characteristics. In other words, 20 or 30 years ago, a person might have gotten away with leaving a motorcycle to sit with fuel in tank and carb for extended periods of time. Many years ago, the writer of this article had a motorcycle that sat for over 10 YEARS with old fuel. Tires were flat from sitting so long, but the engine still started up and ran fine! BUT, that was a long time ago. Times have changed! Today's fuel can deteriorate in a matter of MONTHS. When it does, it can break down and leave deposits that can mess up your carburetor. Deposits left behind by the deteriorated fuel can make the inlet needle valve stick open so that fuel will overflow the carb float bowl. As another example, the author of this article had an engine that was working fine. Then it sat unused for a few months during the winter. When the gas tank was filled in the Spring, fuel began to leak all over. The float inlet needle valve was stuck open by the junk fuel deposits left by the "modern" gas. These reformulated fuels might burn "cleaner", but what about all the emissions caused by improperly running engines that have carbs gummed up by deposits?!? What about all the gas that overflows on the ground (sometimes unnoticed until the tank runs empty), because the newer junk gas gums up carburetors so easily?!? Oh well, it's all in the name of progress. :( Give me that outdated 30 year old fuel formulation any day! The solution to the gummed up carburetor is again: disassembly and cleaning. Adding a fuel stabilizer like Sta-bil should help slow down the deterioration of stored fuel. You add the stabilizer along with fresh gas. Run the engine long enough to be sure that the treated fuel has filled up the carburetor. It won't do much good to add stabilizer to the tank and run the engine too short of a time so that the treated fuel never makes it's way through the fuel lines and into the carburetor float bowl. If you run the engine for around 5-10 minutes, then that should be safe. Alternatively, some people prefer to skip the fuel stabilizer, drain the tank, and then just run the engine until the float bowl runs empty. This can work as long as you are pretty sure that float bowl is actually empty. You can also remove the drain screw on the bottom of the float bowl and drain the bowl that way. Easiest way is probably to use a fuel stabilizer. So if your Yamaha dirt bike carb leaks fuel from overflow tube, then you might need to take apart your carburetor, hope that you can get it cleaned out, and then next time be more aware of the junk fuel with which we now have to live.
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Carb Leaks Fuel