FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician is troubleshooting a reciprocating engine that exhibits consistent backfiring through the carburetor, especially during acceleration. Which of the following is the most likely cause?
- ALeaking exhaust valve.
- BIncorrect ignition timing (too advanced).
- CExcessively rich fuel/air mixture.
- DClogged fuel filter.
Show answer & explanationAnswer & explanation
Correct answer: A. Leaking exhaust valve.
Backfiring through the carburetor (intake manifold) on acceleration is often caused by an intake valve that is not seating properly or an exhaust valve that is leaking, allowing hot exhaust gases to ignite the incoming fuel/air mixture. A leaking exhaust valve is a common culprit.
Why the other options are wrong
- B. Incorrect ignition timing (too advanced) would typically lead to pre-ignition or detonation, not backfiring through the carburetor.
- C. An excessively rich mixture would more likely cause black smoke, rough running, or a decrease in power, not typically backfiring through the carburetor.
- D. A clogged fuel filter would cause fuel starvation, leading to a lean mixture, power loss, or engine stoppage, not backfiring through the carburetor.
Backfiring (Carburetor)
An explosion or combustion occurring in the intake manifold or carburetor of a reciprocating engine.
- Often caused by a lean mixture or valve issues.
- Indicates ignition of fuel/air mixture before or during intake.
- Can damage induction system components.
Memory trick: Valves Are Often Leaky, Causing Intake Combustion.