1. Spark plug damaged; Spark plug loose; Faulty coil or condenser; Incorrect spark plug; Choke needs adjusting; Improper fuel and oil mixture; Dirty carburetor filter; Distributor cap cracked.

2. Backfiring can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet