You did not state how old the engine was. Is it pre-red-head, or early red-head? Does it have the film canister ignition, or the flat package Vlach? Single or double magnet?

The pressure line should be void of fuel.

Check your ignition timing. It should fire in top dead centre(Vlach) , or just slightly before that. The film can ignition should fire 15 degrees before dead centre. The sensor must just cover the magnet, and should have the longest part to the magnet collar, without being twisted out of that orientation. The gap between sensor and magnet should not exceed 0.6mm (.025"). Check voltage on the ignition terminals. It should at all times be more than 4.5 volts. Set spark plug gap to .020" (0.5 - 0.6mm)
If all is right, and the engine still backfires, have your ignition checked by your dealer, or replace it with a new one. Depending on engine age, you may have to replace the prop drive washer.