Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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Have you tried replacing the spark plug? Did it make any difference?
The other thing that comes to mind is a possible bent push rod which would cause a backfire.
Had the engine been running okay? What happened the last time that it was used if anything that caused it to quit running?
I would check the push rods in the engine, it sounds like one of them may be bent or has come loose.