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unexpected rev

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We recently had head work done on my 06 yz250f, while it was apart we cleaned out the carb. Anyways when giving it a consistant 1/8th of throttle the bike revs steady then after about 1-2 seconds revs higher when the throttle remains constant. Anyone have any ideas on what to check first?

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its a carb issue..most likely the Pilot circuit...ideally u want the bike to rev high, with a full twist of the throttle, and when u let go , u dnt want the rev's to come back down very zingy or erratic..it sould come back down to the right rpm's during idle....the main fuel/air metering systems in the pilot circuit would be the fuel screw, pilot jet, idle screw...

to adjust it with ur ear

with the bike warm and idleing turn the fuel screw in till the idle drops/misses.then go back out till the idle peaks/smooths.

this should happen between 1 and 2.5 turns on a fcr carb and 1 and 3 turns on a cv.

if you end up at less than 1 turn you need a smaller pilot jet.more than 2.5 (or 3 turns on a cv) you need a bigger pilot jet.

choose the appropriate size and retest.

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Sorry let me clarify my first post. When you give it a consistent 1/8th throttle it revs as is should but after 4-5 seconds it will rev to the moon for no apparent reason, while the throttle hasnt moved at all. We have determined it not to be a carb problem because we switched out the carb with another one and the problem is still the same. We did have the crank rebalanced and we think it is an air leak? Anyone else ever have this problem? Ignition maybe? This has never happened until the head work/ crank work was done one the bike.

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It could be an air leak. You can check this by spraying water around the intake boot etc. If there is a leak the rpm will change when water gets sucked in. Then you can pinpoint the leak and repair it.

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