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Misfire

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I have a 07 yzf 250 that has a slight misfire at the top end. I have changed the gearing to 14 - 49, still no change, and gone up two sizes of MJ to 182 with still no change.

SPEC.

crower cams

high comp. piston

twin air powerflow

gas flowed head and piston

quick shot

wet sump

hgs pipe

The jetting of 178 MJ and gearing of 14 - 49 were fine. Then at a sand track i went up to 180 and didn,t notice a misfire. The track yesterday was soil slightly sticky with some long straights. It was only misfiring at top end. My tuner has said maybe the Crower cams are producing some valve bounce.

Just wondered if you guys have any thoughts

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How high of RPM are you talking. Normally a misfire at high RPMs is the rev limiter kicking in. If you are not high enough for the rev limiter to kick in then it is most likely an incorrect main jet. Too rich of a main is hard to notice other than the bike will not want to rev out. Too lean of a main jet will cause normally it to pop and spit at high RPM's.

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My sons the rider, not me. He said he had it pretty well pinned open, but it doesn,t sound like that popping and banging of the limiter being hit, but i changed the gearing to see, and it made no difference. Thats why i went for the main jet change.

Plug readings previous to this have looked fine, so i wouldn't have thought that it needed to go any higher than 182, but i suppose i ought to give it a go until it causes a noticable slow down just to rule it out.

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