I recently bought a Bear Tracker that is idling rough and stumbles when giving it throttle. If I let it warm up enough I can eventually work the throttle to higher revs. When you first put in 1st gear it goes very slowly maybe 2mph at full throttle until it works it's way up to finally kicking into full power after about 30 seconds of putting around and then runs great in higher gears/higher rpms. When you put it back into first gear and allow it to go back to lower rpms it stumbles again. Sometimes it will hang on and idle at a higher rpm range which will allow it to run great but when it drops back to the lower rpms it starts to stumble again. It also pops through the carb/intake boot at low speeds and I have a glowing red exhaust.

My suspicion was that it is running lean so I changed the pilot jet from a 17.5 stock to a 22.5 and still have the same symptoms. I thought there may be a vacuum leak but I can't find anything. I have verified the float is set to the correct height, timing is lined up, compression is good, valves are in spec, fresh gas, and the carb is new. What am I missing, Any ideas?

 

I once had a KLF300 with a cherry red exhaust, it was the cam "gone soft" and not lifting the exhaust valve much. Plug colour should tell you if it is rich or lean.