first of make sure the carbureter is the problem. OLD SCHOOL TUNEUP. check compression, check distributor bushings(shaft play), check dwell, check condition of coil and plug wires, check timing, i have some old carbureter books that explain this. if you can't find a kit for your let me know and i'll look up for you .
he is correct. check everything else first. carbs are usually the victim of backfiring not the cause. if it is real cold where you are and the choke is wide open this will cause a backfire.
98% of all backfiring is electrical. Check your ignition system and timing? other 2% is mechanical and/or may be carburetor tune. Check valve clearances and/or for flat cam?