Question about Yamaha YZ 250 Motorcycles
If you have a good fat spark and timing is stock, with good compression, your problem is likely fuel/air mixture. Assuming you have good quality fresh fuel/oil. Set air mixture screw one and a half turns out. What is the condition of the reeds and reed holders. Is there a build-up of fuel/oil residue on the piston port skirts or the crankshaft? Check for absolute seal between carby and reed valve manifold. Check for obstructions through airbox inlet, filter and inlet to carby. Ensure the governor is not fouling. The site link below has exploded view diagrams for all components of the bike to see how it all works. Good luck, cheers.
Posted on Jul 16, 2010
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Posted on Jan 02, 2017
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