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http://en.wikipedia.org/wiki/Back-fire
A Back-fire or backfire is an explosion of a car's exhaust, which creates a loud noise and which can sometimes even result in the emission of flames from the vehicle's tailpipe. The term was derived from experiences with early unreliable guns which could literally blow up in a shooter's face. From this came the use of the word "backfire", as a verb to indicate something that produces an unexpected, undesired result.
Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio. Usually, backfiring occurs in carbureted engines that are running lean where the air fuel mixture has insufficient fuel. ("Running lean" is typically a sign of mal-adjusted carburetors or fuel injection where there is not enough fuel for the amount of air). Afterfire, however, occurs in engines that have an exhaust leak or unburnt fuel in a decatted exhaust system. When a driver shifts up and lets off the gas, the engine has a moment of running rich or with insufficient oxygen. This causes an incomplete burn which causes the fumes to explode in the exhaust system. The leak itself is the most dangerous aspect. Without it, the mixture would cool enough not to explode. A fuel injected engine may backfire if an intake leak is present (causing the engine to run rich), or a fuel injection component such as an air-flow sensor is defective.
Common causes of backfires are:
Timing If car is distributor-less
Timing If a two stroke engine is backfiring every other stroke.
Fuel pressure, fuel filter and pump leaks and corrosion could cause lack on fuel injection.
Bad wiring in ignition ..if car won't run for more than a few moments.
For some engines, backfiring may be considered normal. Backfire is rare in modern cars with fuel-injection and computer-controlled fuel mixtures.
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