If you've lashed the valves to spec (.012" intake, .018" exhaust) my guess is that the accelerator pump membrane may stick. it is made of leather and doesn't like gas with ethanol in it (caused mine to stick)
Accelerating suddenly without the accelerator pump causes a sudden lean condition with could result in a backfire.
Check if yours works by manually operating the accelerator pump and observe for gas squirting into the carb venturi. Do this when the engine is not running or you might loose some facial hair
