What causes an engine to backfire?

Timing is off. You may need a new timing belt.

somtimes usually not

Sounds like the valves are not closing properly. Something in the valve train is damaged. Could be a bent or broken valve, broken valve spring, broken or bent rocker arm, bent push rod, etc. Low...

Your Timing is not lined up properly and/or re-jet your carb.

backfires occur when not all the fuel is ignited in the engine and instead ignites in the exhaust.