During a runup, a pilot notes an excessive magneto RPM drop on the left magneto that clears up after leaning the mixture briefly at higher power. What is the most probable cause of this condition?
- ALead fouling of one or more spark plugs caused by prolonged operation at a rich mixture and low power
- BIncorrect internal timing (E-gap) of the left magneto
- CFailure of the impulse coupling on the left magneto
- DDetonation occurring in one or more cylinders
Show answer & explanationAnswer & explanation
Correct answer: A. Lead fouling of one or more spark plugs caused by prolonged operation at a rich mixture and low power
Extended ground operation at low power with a rich mixture allows lead deposits from the fuel to build up on spark plug electrodes, fouling them and causing a rough magneto check. Leaning the mixture and increasing power momentarily raises combustion temperature enough to burn off the deposits, clearing the fouling and restoring a normal magneto drop.
Why the other options are wrong
- B. Incorrect E-gap timing would cause a consistent drop that does not clear with leaning.
- C. An impulse coupling failure affects starting, not a cleared runup drop that responds to leaning.
- D. Detonation produces a knocking/rough-running symptom, not specifically a magneto check drop that clears with leaning.
Spark Plug Lead Fouling
Deposits of lead compounds from leaded avgas that accumulate on spark plug electrodes during rich, low-power operation, causing rough running or magneto drop that can often be cleared by leaning and increasing power.
- Common during prolonged ground idle/taxi
- Cleared by leaning mixture and brief high power
- Chronic fouling requires spark plug cleaning/replacement
Memory trick: Lead builds up like soot on a candle wick—burn it off by leaning and revving.