As is implied by the term "combustion engine," a small engine creates and contains a series of small explosions within its combustion chamber. Sometimes, these explosions can occur outside of this chamber, such as in the engine intake valves, or in the exhaust system connected to the engine. This can occur if fuel begins burning inside the engine before the intake valves have fully closed, but it can also occur if fuel finds its way into the exhaust system and collects in small pockets. When these explosions occur, a loud bang will typically be heard, and it may be possible to see a brief but intense flame emanate from the exhaust pipe.
Causes Related to Fuel Systems
Low fuel pressure can cause engine backfire through the exhaust system. When fuel pressure is low, the ratio of air to fuel inside the combustion chamber becomes too high. Often, this will under-power the engine for a few seconds or more, during which time some of the fuel in the chamber may not become ignited. This fuel blows into the exhaust system, where it can pool and collect at almost any low spot in the exhaust pipes. If any slight spark, or even sufficient friction, should ignite this fuel, it will cause a backfire. Low fuel pressure can be caused by a failing fuel pump or fuel injector or an old fuel filter.
Causes Related to Engine Timing
Combustion engines are calibrated to generate sparks, and therefore combustion, on a specific timing schedule. This engine timing is designed to work in concert with the intake valves, which close off the combustion chamber of the engine. If the timing of an engine becomes thrown off, it becomes possible for the engine to produce a spark before the intake valves have become fully closed. When this happens, a backfire can be produced through this valve. On modern engines, this type of backfire is particularly unlikely, because the timing is typically controlled by a computer. On older cars with mechanical engine timing and ignition systems, the timing can become unregulated with time, creating the potential for backfires. An engine tuneup may correct these issues.
