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I'd be checking the fuel delivery system, and the ignition. A new spark plug might just fix your problem. The only reason I can think of as to why the exhaust valve won't hold it's setting is valve seat recession. I'd be checking compression before tearing into the engine. Check things like the big rubber intake manifold for cracks, fuel filter, clogged main jet, petcock operating as it should, gas tank venting. The popping sounds to me indicates an ignition problem or lean fuel mixture. If it is the exhaust valve or valve seat causing the problem I'd just get a new head assembly and change that out and then make sure the motor isn't running to lean and causing the motor to run to hot leading to early valve failure. Let us know what you find and how you fixed it.
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