I have a 2003 scrambler 500 and i am having some trouble. I was riding last summer and all of a sudden it started to backfire and there was no power at all. The more gas I gave it, the worse the backfire gets. It will idle just fine and runs great going about 5-10MPH. We went to the bar to let it cool down and after about an hour I started it back up and it ran fine for a few miles and then did the exact same thing so something is obviously getting hot to make this happen. After this trip I went through a lot of the ATV and replaced a lot of items which included the air filter and spark plug. I took the head cover off to look at the cam shaft because I had a dealer tell me that the middle lobe will become rounded and what I found was that it that mine was fine and the lobe looked great. I cleaned the carb and made sure that all the electrical connections up front were good. Went to the cabin the following weekend and rode it around fo quite a long time and it ran great from cleaning everything and i could not get it to backfire and it ran great all weekend. The following weekend I went on a longer trip and everything worked great the first morning and then all of a sudden it started to backfire and do everyting described above. I spoke to more dealers and they are thinking that it might be the stator and I would more than appreciate any help can be provided with this problem.

 

Mine acts like that when the pre filter gets dirty