Is it "popping" through the carb? Or backfiring through the exhaust? If it is then i would check out the cam and springs like Dave said, It will backfire through both of them if springs are broken or bad,weak. The engine will run ok low RPM but when it try's to climb higher on RPM the springs cant handle it.Now this is only if you have done all what the other guys have told you to do.
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