FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsEasy
A pilot reports rough engine operation after prolonged ground idling with a rich mixture during a long taxi. Post-flight inspection of the spark plugs shows dry, black, sooty deposits on the electrodes. What condition is this, and what is its primary cause?
- AOil fouling caused by worn piston rings allowing oil into the combustion chamber
- BCarbon fouling caused by an excessively rich mixture and extended low-power operation
- CLead fouling caused by operation at low power with leaded fuel
- DMechanical fouling caused by a bridged electrode gap from detonation debris
Show answer & explanationAnswer & explanation
Correct answer: B. Carbon fouling caused by an excessively rich mixture and extended low-power operation
Dry, black, sooty deposits are characteristic of carbon fouling, which occurs when the fuel/air mixture is too rich for the power setting, common during extended idling or taxi. Leaning the mixture and briefly running the engine at higher power can help clear light carbon fouling.
Why the other options are wrong
- A. Oil fouling deposits are typically oily/wet and black, not dry soot, and imply an internal engine problem.
- C. Lead fouling typically produces grayish-brown lead oxybromide deposits, not black soot.
- D. Mechanical fouling refers to physical bridging by debris, not chemical deposits from combustion.
Carbon Fouling
Dry, black, sooty deposits on spark plug electrodes caused by an excessively rich mixture, especially during extended idling.
- Most common at idle/low power with rich mixtures.
- Deposits are dry black soot, unlike the oily deposits of oil fouling.
- Can often be cleared by leaning and running the engine at higher power.
Memory trick: Carbon is Coal-black from a Choked-rich idle.