A carburetor's constuction and operation is based on some basic physical principles - dynamics of fluids.
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| the flow of fluid through the Venturi tube |
The image above shows the flow of fluid through the Venturi tube which has bigger diameter Ф1 aperture on input, and smaller diameter Φ2 aperture on exit. Because of different tube diameters, the fluid flows faster through the tube with smaller diameter and as a consequence of that is that the pressure on the wall of the narrower tube p2 is smaller than the pressure p1 on the wall of the wider tube (Bernoulli's principle in fluid dynamics). The result of that is that one has fluid suction from the pipe with lower pressure p2. This physical principle is exploited in carburetor for suction of fuel from fuel chamber and making of mixture.
A backfiring on deceleration is generally caused by a lean mixture in the pilot circuit. What happens is that the mixture leans out enough to where is fails to ignite consistenty. This, in turn allows some unburnt fuel to get into the exhaust pipes. Then when the engine does fire, these unburnt 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. In order to avoid backfiring check the pilot circuits, and set it a bit richer.
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