If you don't find any issues with the ignition or carburetor then possible weak valve springs. Had a 147 years ago that would idle perfect up to bout 1/4 throttle but 3/4 to full throttle it would backfire through the carb. Problem was the intake valve spring was very weak and would allow the valve to start floating and not closing tight at full throttle. New springs and lapped the valves and no more backfire
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