Some of the things that come to mind that could cause popping back through
the carb. are:
cross firing because of improperly run spark plug wires or cracked or worn
plug wire insulation....
distributor cap cracked or arcing between wire towers...
intake valve not sealing properly because one is burnt or one might be bent
if you floated the valves with too much rpm, or one valve is adjusted too
tight ( if solid lifters are being used ).....
possibly a broken intake valve spring.
Kind of a crummy choice of causes isn't it? I would hope for the plug wires
to be the culprit!
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