i use a NGK B9ES. I've never tried anything different because that is what came in my quad when i bought it used. I figure if it runs good, why change the plug type? I've owned my 250r since feb 8th and i have NEVER fowled a plug. I almost fowled one when i was breaking the engine back in from the rebuild...but i replaced it and still no problems. Good luck...i'd try your jetting.
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