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Re: Engine miss at constant speed - Could it be the CARB?

From: Hank Dozier
Email: dozierhc <at>
Remote Name:
Date: November 16, 2003


If you have access to a vacuum pump and a decent vacuum gauge, warm the engine up and at idle, disconnect the vacuum advance. Record the idle timing. Then apply slowly vacuum to the diaphragm, and record the timing increase with each inch of vacuum increase, until it stops. Plot this out on graph paper, and compare to the Service Manual book value for your engine to see if it is the same. This will also test to see if the diaphragm holds vacuum as well. If this turns out OK, then leave the vacuum advance disconnected, and slowly rev up your engine and do the same (every 250 rpm) up to about 4000 for the governor (mechanical) advance. Hold it at speed for a bit, to see if slop in the weights or sticking is causing fluctuating timing. It could be that you have some wear in the distributor shaft, or that the grease in the weight pivots is causing some stiction.


Last changed: July 19, 2018