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99 Yamaha 25 two stroke surging on top end

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av8erdunn

New member
I have a 99 Yamaha 25 two stroke that's surging on top end. I have rebuilt the oil system, taken the carb apart numerous times while cleaning the Jets, float needle valve and flushing every port using alcohol. After all that it's still surging on top end. I Can start to feel it at a little more than half throttle and it gets worse the more throttle I give it. It surges every couple seconds and drops maybe 100-150 rpm if I had to guess. The hose is new and the fuel tank is new. Fuel filter is clean and spark plugs are new. What am I missing? Other than surging she runs great.

Thanks in advance.
 
I know it's an old post. I have the exact thing with the exact motor, can't figure it out. Must be sucking air from somewhere?
The surge doesn't change as you pump the ball, even tied different tank and hose, same thing.

Oil injection has been deleted on this motor and it's running premix.

Thanks
Mike
 
Check your high speed mixture adjustment on both carbs. On mine the knurled end of the adjustment screw had popped off and it let the mixture slowly back out until it fell out which created a leak. I lucked up and found it laying in the bottom of the cowl. Bought a new screw and spring, adjusted and she was good to go.
 
My problem unfortunately isn't the screws. I've had these 2 carbs apart probably 5 times thinking there was crap plugging passages. I've had both of them completely apart and dipped in carb gunk and another time completely apart and cleaned in an ultrasonic cleaning tank.

Thanks for the thought though, keep them coming.

Mike
 
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