Q: Revs breaking up at mid range and running very rich

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Hi, so I have Honda clone 70cc Fourstroke motorbike that I built as a school project, the issues I am having include the revs breaking up at about half throttle and about half way through the rev range, I also am having a big problem with it running very rich resulting in fouled spark plugs, I have however had the carb apart and have the taper needle jet set to as lean as it can be but still having these two issues, ocassionally the bike will stall and sometimes will blow out faint black/grey smoke, and runs much better when its cold. Was wondering if you could tell me if these two are link and possible resolutions to the problems cheers

You may also want to check to be sure the float in the carb is not sunk. Many times on these small motorcycles, especially if they have been sitting for any length of time, the carb floats can develop very small pinholes in them causing them to fill with fuel. As this happens, they end up sinking inside the float bowl, causing it to allow too much fuel in. As the chamber feeds fuel to the carburetor, the fuel level sinks, and the float falls with it. When the float drops below a certain level, it opens a valve allowing fuel into the chamber to refill it from the main gas tank. Once the chamber is full, the float rises, closes the valve, and the fuel feed switches off again. This should lead you to your problem. If you need additional help, please reach out to us here at YourMechanic as we are always here to help.

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