[H. L. Chauvin]

One (1) spark plug not firing now & then, (intermittent fire), is just one cause of backfire.

For example, knew a show-off teenager with an almost perfect Model AA truck who used to always turn the switch off & quickly turn it back on to "make" it backfire.

In this case, unburned fuel mixture is pumped into the exhaust manifold & when this fuel mixture is later ignited in the exhaust manifold, it caused the backfire.

The "Rest of the Story" with his backfiring truck & a murder case is just too long to mention.