Things to Look For:
Backfiring is a subdued explosion in the intake manifold. Causes are:
Lean mixture (often due to dirt or water in fuel).
Engine cold and choke too lean
Leaky or sticking intake valve or weak or broken intake valve spring.
Leakage of current across distributor cap may cause backfire by enabling spark to occur in a cylinder which is on its intake stroke. Two mixed-up spark plug wires may also cause this trouble.
Popping back is synonymous with backfire.