Lou,
there are several things that can lead to backfiring. out of the exhaust, i would look in the unused fuel/air direction. that is the clyinder did not fire at all, and dumped raw fuel and oxygen rich air into the exhaust for ignition from another cylinder's exhaust. look at the plugs, make sure all of them are a nice creamed coffee color. this indicates that they are burning propperly. also make sure your plug gap is not to big and you have a good healthy blue arc. look for hairlined cracks on the insulators around the electrode tip, cracks allow the spark to short away from the fuel/air and not light the cylinder off. then look at the cap, make sure the terminals inside the cap are not burned up or corroded. plug wires are next, they get old an can lead to intermitten misfires. if the backfire is steady, then it may be mechanical like a burnt valve, itermittened is electrical or carb problems. if it does it under accelleration only, then you may have a bad accellerator pump or bad base gasket under the carb. to lean a mixture will cause backfiring at times.
main things are plugs and bad base gasket. that is where i would start. Brian
