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low rpm bog

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I'm here for some serious advice.I ride a 2003 yz250.A few months ago I was @ the track.Than I noticed my bike bogging out of a certain corner.My buddy thinks my crank is going out.I'm confused cause I changed my top end.I have great compression.I'm hoping its a small problem such as a carb issue????

Thanks in advance.....

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My first impression is that its not a crank issue. There's a possibility that there could be a crank seal issue thats causing the bog, but its not likely. Does it smoke excessively when it does it? Also does it eventually climb through the bog into higher RPM? I had similar issure one time with a 125 where the power valve was stuck in the closed position and the engine would not climb into higher RPM. It could be something simple like you were saying like a carb issue, did the bike sit for a long period of time?

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I don't believe it's the power valve.since when I changed the top end.I also changed the cylinder.which involves swapping out the power valve.which I cleaned out.it only seemed to bogg down @ low rpm's in one of the slower corners.other than that.it seemed to have full power....

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Yeah it doesn't sound like the power valve then if it eventually climbs out of the bog. Sounds more like a carburation issue. Did you replace the reeds when you did the top end job? They can start to crack, chip, or not fully seat and cause that problem. Also the slow speed jetting could be off, a partially clogged pilot jet or air bleed or the air screw adjusted improperly. I would check those things if you haven't already.

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