Question about 2003 Yamaha WR 250 F
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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it sounds like its starving for fuel, when it runs lean you will get backfires and if its running rough then it may be starving for fuel at idle. Try adjusting you mixture screw and see it the engine will smoot out. Your carb my be dirty and not allowing enough fuel through it.
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