Yamaha Warrior from 02 - 05 had this problem it turned out that the pulley lock nut was the problem it was fix in 06, but it usually took the shaft with it. Knowing this I check mine for filings or dust whenever I think of it, also when I got it my belt was at 240, I dropped it to 200, after reading all the posts about what it should be I went to 180, didn't think I wanted that much tension on the drive pulley, also my vib went from 55-60 to just a little bit around 70. So if a few have had this problem it would probably be a good idea to check it every so often, if you reach up behind the cover you can feel the pulley if your fingers come out with black dust better check further.