It shouldn't be a bad vacuum advance, my '54 runs just fine without one, except occasionally when ya hit a bump and your foot hits the pedal too hard, then it stumbles a bit, but let off of the pedal again and all is fine. My yard is big enough to get the engine speed up(in first) without being in danger and the only thing the vacuum advance does to my 235 with a Carter Y-F(Yes I know two different engines, and carburetors too) is make the two gallons in the gas can disappear faster. But at least you are buying something that needs replaced. Here's something I thought of, not sure how safe it is, but drive around until it does the backfiring, then take it home, pop one of the sparkplug wires off and see if they arc when you have someone turn the engine over. Don't know how but you may be able to do so in a safer way? If not then don't try it if it isn't safe, don't need anyone getting hurt.
