Something is making it run rich. It was smoking from being rich and it backfired from being rich. The "backfire" is from unburned fuel in the exhaust pipe that gets ignited from the hot gases entering the pipe. You may be having trouble staring it now because the plugs are fouled and the crankcase may be flooded. Remove the plugs and pull it over to help pump the crankcase dry and get some new plugs, probably a bunch because you may fouling them out until you find out what is wrong. One of the first things to try with the old EFI's is a new battery. They are very sensitive to battery condition.