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Aircraft systems and instrumentsQuestion 610 of 934

A rich mixture at high altitude can cause rough engine operation. What is the usual cause of the roughness?

  1. Spark plug fouling from excessive carbon buildup
  2. Detonation caused by a mixture that is too lean
  3. Carburetor ice from the cold, moist air aloft
  4. Worn piston rings letting oil into the cylinders
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Correct answer: Spark plug fouling from excessive carbon buildup

A rich mixture lowers the temperature in the cylinder and prevents complete combustion, so carbon builds up on the plugs.

Source: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 7: Aircraft Systems — Induction Systems — Mixture Control, page 169

Practice aircraft systems and instruments

Questions last reviewed: 18 October 2026