Alan sounds like we could ask are we having fun yet? You have certainly got a lot of very good help from these posts. After I read thru them, knowing you have a running engine that backfires, it reminded me of a game some of us did in a younger part of our life. We would turn the ingnition off for a few seconds and then back on as we were driving along. Guess what happened,
Yes in exploded the fuel buildup, destroyed the muffler, it swelled to the floor beating size. Is it possible that the Ignition switch/wireing, is turning of and on very fast? How to find out. Hot wire it, Take a piece of wire from the positive post of the batt. If it has one in the system put in a ballast resistor Attach it to the + side of the coil, takeing the other stuff off and see what happens. If it still backfires the problem is in the Dist. If not it is in the switch/wireing leading to the Dist. Just a thought, hope it helps.....Chb46