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 fuel mixture

2. Excessive oil level

3. Lean mixture

4. Incorrect ignition timing

The correct answer is: Lean mixture

A reciprocating engine backfiring through the induction system at low RPM is most likely caused by a lean fuel mixture. When the mixture is too lean, there is insufficient fuel relative to air in the combustion chamber, which can result in incomplete combustion. This incomplete combustion can cause unburned fuel to exit the cylinder and ignite in the induction system, leading to a backfire. At lower RPMs, the engine operates at a slower pace, making it especially sensitive to mixture ratios. If the fuel-to-air ratio is not balanced correctly, the likelihood of backfiring increases. A lean mixture generally means there is more air than fuel, which can create an environment where igniting the mixture becomes erratic. The other options, while potentially contributing to engine performance issues, do not directly correlate to the specific phenomenon of backfiring through the induction system at low RPM. A rich fuel mixture might lead to rough running but would be less likely to cause a backfire under those conditions. Excessive oil level affects lubrication but does not typically influence backfiring. Incorrect ignition timing can cause backfires as well, but it more commonly leads to issues at higher RPM or varied operating conditions, rather than specifically at low RPM.