Why backfire occure in gas engine ?

A backfire is an explosion produced by a running internal combustion engine that occurs in the air intake or exhaust system. Backfire may cause an objectionable popping noise with possible loss of power and forward motion.

A backfire can occur  in the intake manifold, carburetor, or air filter of an internal combustion engine when the intake valves are not shut prior to fuel combustion.

Also a backfire can occur in exhaust system when the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion. The result of a misfire or incomplete combustion is that unburned fuel delivered to the exhaust manifold where they may ignite. Another backfire situation occurs when the engine is running rich with excess fuel and there is incomplete combustion with similar results.

Common causes of backfires are:

1. Poor or unregulated engine timing is often a cause of backfires in both intake and exhaust system.
2. Improper wiring in the ignition can also lead to timing issues and backfires.
3. Low fuel pressure, clogged fuel filters, and weak fuel pumps (Turbocharger pressure) could cause a severe lean air-to-fuel ratio during fuel injection and cause backfire.

We faced backfire problem in our gas generator engine with big pop sound and air filter breakage. So backfire occurred fuel intake system due to lack of improper intake valve timing or intake valves may stick due to carbon deposits in the valve guides or valve seats may be uneven for which valves were not close properly. 

For which it is necessary to clean all carbon deposited in cylinder head and grinding valve seats to make evenness to seat valves properly.

Also may check total electrical systems which are related to engine ignition system like spark plugs, ignition harnesses, ignition modules etc.