While checking the timing as I was troubleshooting the backfiring problem, I wondered if the timing mark plate may have moved. I decided to check if top dead center was really lined up with the 0° advance timing mark. This simple device screws in place of the 1A spark plug. Using a 7/8 wrench, I turned the engine clockwise while my wife watched the plunger rise. It stopped going up at 6° BTDC and started to go back down at 6° ATDC, so it looks like the veneer has not been moved and is in the right spot.
