the door latch on the body may need adjustment. Check timing first, that is what the backfiring sounds like, also change the plugs, they may be fouled by now and it will drive you crazy trying other things before you realize it is the plugs.My experience is that even poorly tuned 4 barrel carbs do not cause a lot of backfiring, webers are another story, so electrical is likely the culprit. Start your timing at 12 degrees BTDC and go from there, make sure advance is working.