A backfire or afterburn is combustion or an explosion produced by a running internal combustion engine that occurs in the exhaust system, rather than inside the combustion chamber. It is also sometimes referred to as an afterfire, especially in cases where the word backfire is used to mean a fuel burn that occurs while an intake valve is open, causing the fire to move backward through the system and out through the intake instead of the exhaust. When the flame moves backward it may also be called a "pop-back." A backfire can be caused either by ignition that happens with an exhaust valve open or unburnt fuel making its way in the hot exhaust system. A visible flame may momentarily shoot out of the exhaust pipe. A backfire is often a sign that the engine is improperly-tuned. The term derives from parallel experiences with early unreliable firearms or ammunition in which the explosive force was directed out at the breech instead of the muzzle. That is the origin of the use of "backfire" to indicate producing an unintended, unexpected, and undesired result. Explanation Common causes of backfire are running rich (too much fuel going into cylinders) or faulty ignition, possibly a fouled (dirty) spark plug, coil, or plug wire. Pop-backs are usually caused by problems with timing. If the timing is too early, the spark plug fires before the intake valves close, causing the combustion to propagate into the intake manifold, further igniting the air-fuel mixture there; the resulting explosion then travels out of the carburetor and air filter. On many small marine engines, 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, causing a fire or explosion. Alternatively, the ignition timing may be late, in which case the combustion is not completed by the time the exhaust valves open, allowing the combustion to propagate into the exhaust system. Additionally, 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 popback. In this situation it is conceivable that there is a popback occurring