If you’ve done all that you’ve stated, i would locate the lean/rich adjustment screw on your carb. If mikuni it will be on the right side of the carb towards the bottom (if you’re looking at the front of the motor). The adjustment screw will have a spring behind it. Crank the motor, let it warm up. Make sure your idle is set anywhere between 8-900 rpms. Turn the adjustment screw, minute turns clockwise, until you hear the engine cut back while giving it full throttle intermittently. Then turn the adjustment screw back counter clockwise until you find your throttle response is spot on and there’s no back firing. Sounds to me that the vibration may have you running a little too rich. Pour some sea foam directly into your carb, just a little bit. Do the exact same as stated if you’ve got stock carb.... what did you set your valves to?