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Yamaha 250 will not rev past 4000

Travis Herrman

New member
I have a Yamaha 250 ox66. I got it from a rebuilder. It will start and idle all day long. You put it in gear and give it the gas and it will go to 4000 rpm then drop to 3000 then drop to 1500. Then if you unplug the knock sensor it will hold 4000 all day without dropping but will not go over 4000. What do I have going on here?
 
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