I've been racing my 06 WR450 on the ice all winter, but it has developed a serious top end sputter. It feels and sounds like a rev limiter is kicking in at ~8000rpm and it just won't rev any higher and the power seriously drops off. I haven't changed anything since last weekends ride (same tank of gas- 1 week old, then topped up with fresh today after it started misfiring) and the jetting is the same as always. It has run fine on both colder and warmer days than today. It started out really subtle, but got worse and worse every lap. I made it do it for a few seconds then did a plug chop. It looked pretty white, so I upped the main from a 165 to a 180, which made no difference. I also just checked the valves and changed the oil, and nothing is out of the ordinary. I don't know what to check next, and thought I should ask since a search didn't help much.