Question about 2003 Yamaha WR 250 F

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Fuel Problem Starts-idles fine. Push to wide open throtle and it boggs, then backfires through the intake(carb) and dies (like fuel starved-lean). The short story. I have an identical unit and it runs fine. Used it's carb-same problem. New plug, valves were fine at .002'' and .004'' as per factory. Compression is 160 lbs. Rebuilt the carb early but didnt find anything. Been around this stuff a long time but this one has me baffeled. Oh yea, I blew into the carb vent line as it started to bog and it momentailry picked up. Pretty sure it's lean. Carb is the butter fly/ retangular slide Mikuni.

Posted by Anonymous on

  • Anonymous Dec 30, 2009

    No, I have not. I believe this is a static check where you allign the flywheel with a pointer? Or can this be done with a light? I will look in the book tonight and see. Thanks - Jim

  • Adrian Ahrens
    Adrian Ahrens May 11, 2010

    Have you checked your timing?

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Jeff Hagemann

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If you have 2 carbs that work on another unit but not this one then that narrows down the fuel problem to either an intake/ vacuum leak or bad fuel in the tank.
The compression sounds a tad high which could indicate valve timing ( exhaust opening late or not long enough)
On the ignition side you could have a bad CDI , weak pulser coil or sheared flywheel key.
I believe I would lean toward suspecting the ignition. I had a Honda that gave me fits acting like a lean issue. I was in that carb 25 times if I was in it once. Same thing, idled fine but died on acceleration. It was the CDI. Put a spark tester inline with your spark plug and see if the unit dies due to lack of spark.

Posted on Dec 30, 2009

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