Motor is popping back through the carb... Help!

flynbyu2 said

I know you said the marks are aligned, but this would be my first guess.

The other distinct possibility is that you might have a couple of the plug wires mixed up.

Popping through the exhaust happens when the plug fires when the exhaust valve is open so something is definately wrong there.

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Timing marks were dead on and I checked this twice. The first time when I installed cam and the second time when i pulled engine apart to change oil pump and shaft and check for anything on the engine affecting oil pressure. Timing marks are dead on.