I'd start with a bad plug, a bad plug wire, or a bad terminal in the cap. I've seen old wires short to ground where they lay against other stuff. If the spark can't get to the plug, it will gas foul it.

If it had port injection, I would also be looking at that injector, but a carb/throttle body would usually gas foul all the plugs, or all the plugs on the same intake plane. Gene