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Discussion Starter · #1 ·
I'm getting a fairly consistent momentary loss of power and spit of air from the carb when riding at 3/4 - full throttle.

I didn't put the motor together but its really strong through all other ranges - it is a pre unit 650 with a 750 head running a 930 with a 280 main jet and a joe hunt magneto.

Could this possibly be a symptom of timing or would it more than likely be jetting?

If it in, your opinion a jetting issue, what kind of increase in main jet size would you recommend?
 

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I would need to clarify a few points to begin to analyze your problem. When you say 3/4 to full throttle, do you mean at high RPM, like 6-7 thousand, or is it happening while opening the throttle at lower rpm. Also , you say you have a 750 head on a pre-unit 650. do you mean that you have the entire 750 top end installed on the pre-unit, or only the head. A 750 head installed on a 650 cylinder would drastically lower compression and performance. The 280 main jet in a 930 concentric should be adequate, if not a bit too rich. Please give us some more details.
 
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