Hey there. I'll try and explain this as quick as possible and hopefully I can get some responses from some of you. This is my second year with my XC 600 and this year I put on a rumble pack cannister and did some clutch work and a new 1 1/4" track, it ran good for about the first 2oo miles and then started backfiring in the midrange. It gradually got worse and does it all of the time in the midrange. It idles great and wide open it runs great. I've checked the carbs ( clean ) reeds ( no cracks ) plugs are fine ( changed to a hotter plug ), changed plug caps, and have played with the air screws on the carbs ( right now I have them a 1/4 of a turn out and seems to have helped a bit but not much ) Also it is not the CDI. Any ideas or info would be great. Polaris is my way out!!