rode my sled hard today, it is new to me. near the end of the day, it starting running poor at high rpm. i checked the plugs and one of them was carbon fouled. I put a new set in and it ran good for a bit, then started hesitating again, checked the plugs and the same thing, one plug carbon fouled. Any ideas on what is causing this? I will pull it into the garage tomorrow and check compression. When it started doing this was after coming down a pretty good size mountain and I was idling and braking a lot, could this have something to do with the fouling?