318 backfired and now runs like it has a misfire

It belched black through the carb? I would guess carbon. A old trick I used to do. Warm the engine up to temp, open the throttle 1/4 to 1/3 of the way open, (2500 rpm or so) and trickle a pop can or bottle of water down the carb. TRICKLE
is the key. You can use fuel injection cleaner as well. You may be surprised all the crap that blows out of the tail pipe.