Yep I think its fuel too. I went out yesterday with my new body work ( which looks SWEET thanks TYGA ) Anyway the tank that came with had a little crap in it that I tried many methods to remove and while I got most I don't think I got it all. My bike was doing the same and then it got really bad and I switched to reserve and it was back to just a little sputtering. Sorry for the novel but yeah I think we both have a fuel problem.
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1995 A kit TZ 250
1994 Rothmans NSR 300sp SOLD
1998 RM 250 freestyle/motorcross
1998 Meathanol burning CR 500 AF super retard/motorcross/sand dunes/play bike