I have two 850's a 20 Khaos and 21 RMK and both are dirty running plug foulers. The 20 started life with a GGB can and had plenty of fouled plugs, usually on the PTO side after a cold start. It has the newest Polaris flash on it and I went back with the factory can it still fouls plugs. The 21 is all stock motor wise, including the can and I can almost guarantee a plug swap after each ride. They both run amazing when out riding, the issue is only on cold starts in the morning. I let the sleds completely warm up before touching the throttle but as soon as you do they instantly foul the PTO side plug. I've recently swapped the 21 to the EYA plugs and going to see how that works out. It's frustrating to change plugs every ride but at least it doesn't affect the overall rideability of the sleds when it counts. Both sleds are powerhouses and I don't feel like its a cylinder issue like some here believe.