My friend has a 88 250x it runs great after we put a new plug in it,but after about 1/2 hour it starts to backfire and most of the time will stall.The plug gets really black.The air scew would not make that much of a difference when adjusted in or out.We took the carb. apart and cleaned it and blew it out and still same problem.The only way I could keep it running was to turn the air screw 4 turns out itran ok at low speeds but would backfire and stall if given too much gas.Any replies that might help me out would be greatly appreciated.

 

Well the black plug seems to me that the quad is running to rich. So it may be a jetting issue. But i had some problems at high RPMs with the engine breaking up and sometime shutting off !! I talked to a couple ppl. They all said it sounds like the CDI box. So i went and bought a used (`99) CDI box from a local parts dealer and it knows runs great !! But i don't think it will help your black plug, like i said, it seems like a jetting problem there !!

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Exer_95....Jason....95' Honda 300ex