Jeff Cardinal 7/13/2010probably dirty and maladjusted points. or possibly a cracked distributor cap, maybe old plug wires. check all connections to and from the distributor and coil. The condenser might be dying. I'd buy a set of points and a new condenser. clean the cap and make sure the plug wires are good. Clean or replace the plugs while your'e at it.Backfiring can also be caused by a too lean mixture, check to see that the carb is bolted tight, if an intake system gasket is leaking, it can cause a lean mixture. you can check for intake leaks by spraying WD-40 at suspected places while the engine is running, if the engine stumbles, you've found a leak. Jeff.