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.

What causes Backfiring

• "Deceleration Backfiring" is the most common backfiring that occurs when you change down and throttle off. This extract from the Virago Site explains it succinctly.

  "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 riders have also experienced Backfiring on up-shifting. An exhaust gasket is the most liekly culprit.

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.

Backfiring can be casued by the following:

• Internal carburetor problems (on initial WOT acceleration, this can be caused by a defective accelerator pump).
• Weak or broken valve spring (Usually severe. When it is exhaust, backfire will occur in the intake)
• Low compression
• Cam to crank timing off (rare, usually result in hard/no start condition)
• Burnt/damaged valves or seats (sever usually)
• Worn cam lobes (lack of power, can be severe backfire, often through carb)
• One or more valves adjusted too tight (heard through carb then intake is usually too tight, heard through exhaust then exhaust is too tight)
• Preignition (caused by deposits or lean engine conditions)
• Water in fuel

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).