I can't remember where most of this came from.

Backfiring is in no way related to fuel and a properly tuned engine should not backfire. Backfiring usually occurs during acceleration or slowing down. It is commonly caused by ignition of the air/fuel mixture while it is still in the intake manifold. This usually happens because the inlet valve is sticking or leaking or because there is an ignition system fault. Unburnt air/fuel mixture passing through the exhaust and igniting also results in backfiring. This is usually caused by a temporary loss of ignition or rich idle mixtures in combination with exhaust air leaks. If there is only moderate backfiring it may be because the exhaust valves are leaking or the tappets are tight.

Most of the following came from - Backfiring FAQ

Now, an engine is not supposed to backfire and, when it is persistent, I am not convinced it is harmless to the engine. If there is an explosion in the exhaust pipe when the cylinder is trying to exhaust its spent combustion products a back pressure is created which interferes with the next cycle. This could result in incomplete extraction, irregular charging with the fresh mixture, and overheating. In addition valves are not designed to seal against pressure from their backsides and a broken valve head rattling around in the combustion chamber at 10,000 rpm engine speed is not something I am desirous of experiencing.

There are quite a number of different causes for backfiring. What this FAQ will try and do is to help you isolate the cause of it.

What causes Backfiring

So, how do I fix it?

  1. Idle screw

    Some folks cure backfiring by turning up the idle slightly. If you have mild backfiring, you can give it a shot.

  2. Idle mixture

    No this is NOT the same thing as the Idle Screw. The Idle Screw just limits the minimum throttle when you take your hand off the throttle. The Idle Mixture screws actually control the air/fuel ratio at idle.

    Many riders find that enriching the idle mixture by backing off the screws by 1/8 to 1/2 a turn will eliminate popping. Enrichen as little as necessary, work in small increments, and test the results before backing out the screw another 1/8 turn.

  3. Exhaust Gaskets

    This is another popular cause of Backfiring.

Most of the following came from - Possible casue of Backfiring

Backfiring can be casued by the following:

  1. check that the valve timing is correct and that the timing chain has not jumped one tooth of the camshaft sprockets (details will depend on the car engine). Ensure also that the valve clearance is not badly out.
  2. check that the ignition timing is neither too advanced nor too retarded and that the correct dwell angle is set with respect to the points opening gap (details depend on the engine).