Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio. Backfiring can occur in carbureted engines that are running lean where the air-fuel mixture has insufficient fuel and whenever the timing is too advanced. As the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion. The condition that causes this is a misfire. The result of a misfire or incomplete combustion is that unburned fuel or flammable hydrocarbons are delivered to the exhaust manifold where they may ignite unpredictably. Another backfire situation occurs when the engine is running rich (with excess fuel) and there is incomplete combustion during the Otto cycle, with similar results.
Popularly, this is conceived of as a sharp report produced by almost any type of engine. However, among engine professionals, the ignition of fuel within the engine exhaust system is an "afterfire" while "backfire" is this same process taking place in the induction system, primarily in internal combustion engines. This distinction is useful when troubleshooting problems.
When starting an engine, timing that is too advanced will fire the spark plug before the intake valve is closed. The flame front will travel back in to the intake manifold, igniting all of that air and fuel as well. The resulting explosion then travels out of the carburetor and air cleaner. A common air filter will allow the gases to escape, but will block the flame front. On many small marine engines, no air filter is used, but a screen is placed over the intake of the carburetor as a flame arrestor to prevent these flames from escaping the intake, and potentially igniting fuel, or fuel vapors in the enclosed sump or bilge of the boat and causing a fire or explosion. Improperly adjusted carburetors that create a lean condition during acceleration can cause the air fuel mixture to burn so slowly, that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire. In this situation it is conceivable that there is a backfire occurring in the intake manifold and exhaust manifold simultaneously.
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