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Interpreting an idle mixture check on an RSA injection engine

Aviation Mechanic Powerplant (AMP) practice question 12 of 20 on Engine Fuel and Fuel Metering Systems. ACS element AM.III.I.R1 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONOn an RSA fuel-injection engine, the mixture control is moved slowly toward idle cutoff and the tachometer rises from 700 rpm to 740 rpm before the speed begins to fall. How should this be interpreted?

  1. AThe idle mixture is too lean and must be enriched
  2. BThe idle mixture is too rich
  3. CThe idle mixture is set correctly
Show the answer and explanation

Correct answer: C. The idle mixture is set correctly

  • A — incorrect On a mixture set too lean the idle speed drops with no gain in speed at all, so a clear rpm rise rules this out.
  • B — incorrect A rise of 40 rpm is inside the acceptable band; an excessively rich idle mixture is the leading cause of spark plug fouling and would not be accepted as satisfactory.
  • C — correct The rise of 40 rpm falls within the 25 to 50 rpm increase that usually indicates a satisfactory idle mixture on an RSA fuel-injection engine.

BACKGROUND

Excessively rich or lean idle mixtures cause incomplete combustion, carbon deposits, and spark plug fouling, and they force high taxi speeds with excessive brake wear. Before checking the idle mixture, the engine is warmed until oil and cylinder head temperatures are normal, and adjustments are always made with cylinder head temperatures at normal values. On an RSA fuel-injection engine the optimum idle setting is rich enough for satisfactory acceleration under all conditions and lean enough to prevent plug fouling or rough operation, and a rise of 25 to 50 rpm as the mixture control is moved toward idle cutoff usually satisfies both. The actual adjustment is made by lengthening or shortening the linkage between the throttle lever and the idle lever. If the mixture is set too lean, the idle speed drops with no gain in speed.

STUDY TIP

Do the subtraction first and compare the result against the published band before judging the setting. Also keep the idle mixture adjustment mentally separate from the idle speed stop, which is reset afterward.

KEY POINTS

  • Warm the engine to normal oil and cylinder head temperatures before checking idle mixture.
  • On RSA injection engines, a 25 to 50 rpm rise toward idle cutoff indicates a satisfactory mixture.
  • A mixture set too lean produces no rpm gain at all, only a drop.

SOURCES

  • amp-powerplant/ch02.txt "Adjusting Idle Mixtures"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.