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My engine is intermittently missing & coughing back through the carburetor.

I've changed the following parts which has had no effect on the problem: distributor including its magnetic pickup coil & rotating magnet, distributor rotor, distributor cap, spark plugs including gapping them, plug wires, ignition module, ignition coil, and carburetor.

I temporarily ran my own independent ignition system wires between the ignition coil, ignition module, distributor, and battery and isolated the ignition system completely from the normal ignition system wiring and charging/voltage regulator system so that the ignition was running off the battery only and not the isolated alternator or voltage regulator and the ignition system used only my temporary wiring.

I can't hear or see any vacuum leaks. I checked the ignition timing and it looks good.

Any ideas of what could be left that would cause this problem?

I can only make guesses and those are: blockage somewhere in the exhaust system or intake manifold, sticking valve, bad valve spring, or maybe I somehow did not hear or feel a vacuum leak.

Anyone got any other ideas?

I've read that vacuum leaks will cause a lean mixture (no argument from me about that, it seems obvious) and that a lean mixture can cause coughing back through the carburetor.

Exactly why would a lean mixture cause carburetor coughing? What exactly would be going on to make that happen?
 

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Vacuum advance hooked up?
Advance curve set up for same?








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HI
is this on the rebuilt motor you've talked about in other posts?? is this first run or a new problem that popped up?
Could you have to tight a valve lash, easy to do on hydrophilic lifters. and as Mad asked whats the timing at and have you used a light to set it..

good luck take care be safe
tim
 
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