Basically:
A backfire is caused by a combustion or explosion that occurs when unburnt fuel in the exhaust system is ignited, even if there is no flame in the exhaust pipe itself. Sometimes a flame can be seen when a car backfires, but mostly you will only hear a loud popping noise, followed by loss of power and forward motion.
That unburnt fuel can be caused by a variety of mechanical problems, and here are some of the most common reasons for a backfire:
Running too rich
If your engine is getting more fuel than it needs, a rich fuel to air ratio is the result. When your car has leftover fuel in the exhaust and the cylinders, that fuel explosively burns and creates a loud popping sound.
Engine timing is incorrect
Specifically, here we mean delayed timing, which causes the backfire. Delayed timing means that the ignition cycle of your engine starts late in the combustion chamber, and ignites the fuel as soon as the exhaust valve is opening instead of waiting for it to be fully open.
Cracked distributor cap
Cars that don’t have ignition coils on their spark plugs have a distributor cap and a wire set that disperse the electrical pulse to the spark plugs. If the cap is cracked, it allows moisture to creep in, which causes the electrical spark to jump to the wrong cylinder, causing a backfire.
Carbon tracking on spark plug wires
Carbon tracking can come into play in a variety of situations, the first situation involving sparks that are generated by wires on a distributor cap, can start to hop from one wire to another very quickly and thus a carbon track is formed which is essentially a shortcut for the spark.
The second most common scenario is when spark plug wires or ignition coils are mounted directly onto the spark plug, causing the electrical spark to split paths and leaving fuel behind in the cylinder. When the next spark is generated, it strikes at the fuel left behind and rapid burn occurs, while the exhaust valve is open and you have another backfire.