bought this quad brand new in 2000, havent really had alot of time to drive it until this year, it hgas just over 400 miles on it as of now. It has a tendancy to backfire when i let off the gas after having it pinned for awhile on the roads out here.. Usually just once, and it dpoesnt disrupt anything. It seems to only happen after letting off the gas from a high speed. I can pin it in the mud and it never backfires, oh and also.. going down hills it likes to backfire a little when i leave it to the engine to slow me down. Has anyone else experienced this? We tend to drive our quads extremely hard out here.. so far the cat has taken everything i have given it without fail, im very impressed with it but this backfiring annoys me ;p

Thanks

 

I had the same problem with my 300, only a lot worse. Basically you have to try a combination of different Main Jets and E-Clip positions in the carb. If the backfiring gets worse it will blow out baffles in the muffler, then it will rattle all the time. I'd bring the bike in and insist your dealer contact AC for any service bulletins about your bike.
Good luck!