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Fired up a freshly rebuilt 289 2-v this weekend. Problem:
Runs smoothly as rpm increases, but coming back to the
idle rpm range the engine starts to vibrate noticeably. Shortblock was professionally rebuilt and I used a known
flexplate and original harmonic balancer. Hoping this is
not an imbalance problem. ( If the motor is unbalanced, would it vibrate at all rpms?) Any input as to possible problem
would be appreciated.... I believe the timing and the carb are
OK.
 

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It takes 3 things to make an engine run: fuel, compression, and ignition. I'd say you have the first 2 licked, leaving the the later. I'd venture that the vacuum advance is at fault. When you say that the engine runs smoothly as rpm increases, is this a slow to moderate rate of increase or rapid? Does it hold the higher RPM smoothly or does holding a RPM degenerate to the vibratory problem like that of decreasing RPM?

Tom Kubler, Long-time Mustang Enthusiast & San Antonio Mustang Club Founder
 
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