Popping thru carb when starting

This could be many things, but I think more likely electrical [ ign ].
If points ign, check gap. Check dist cap & rotor for cracks, arcing. If elec ign, check p/up air gap. No gap or too big a gap can cause backfires. If the vac adv is used, after high mileage the p/up wires break from constant VA movement.

Check plug leads & plugs.