FAA Powerplant Written Practice Test

What is the most likely cause of a reciprocating engine backfiring through the induction system at low RPM?

1. Rich mixture

2. Lean mixture

3. Incorrect ignition timing

4. Clogged fuel filters

The correct answer is: Lean mixture

Backfiring through the induction system in a reciprocating engine, particularly at low RPM, is most commonly associated with a lean fuel mixture. When the fuel-air mixture is too lean, there is insufficient fuel relative to air in the combustion chamber. This can cause incomplete combustion, leading to unburned fuel entering the exhaust system. If this unburned fuel reaches a point where it can ignite (often due to the heat from the exhaust), it can cause a backfire. At low RPM, the engine operates at lower vacuum levels, which can exacerbate the lean condition. The reduced fuel flow can result in a higher likelihood of misfiring or the combustion process not being complete, ultimately leading to backfiring in the induction system. Furthermore, a lean mixture can result in hotter combustion temperatures, which further contributes to the potential for backfiring. In this scenario, while other factors like incorrect ignition timing or issues with the fuel system can also lead to backfiring, a lean mixture is most directly tied to this specific cause of backfiring occurring at low RPM.