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.
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.
"In general, backfiring on deceleration (as opposed to acceleration) is generally caused by a lean condition in the pilot circuit. What happens is that the mixture leans out enough to where is fails to ignite consistently. This, in turn allows some un-burnt fuel to get into the exhaust pipes. Then when the engine does fire, these un-burnt gasses are ignited in the exhaust pipe, causing the backfire. Newer Virago carbs have an 'enricher circuit' which cuts in on deceleration to help this problem. Earlier carbs do not have this. I would say check the pilot circuits, and set them a bit richer. It is hard to do this right without some equipment to test where you are. Aftermarket pipes often increase backfiring, probably because they may the bike run leaner."
Some folks cure backfiring by turning up the idle slightly. If you have mild backfiring, you can give it a shot.
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.
This is another popular cause of Backfiring.
Backfiring can be casued by the following: