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Soaking a diaphragm carburetor filled with fuel

Aviation Mechanic Powerplant (AMP) practice question 15 of 20 on Engine Fuel and Fuel Metering Systems. ACS element AM.III.I.R4 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA carburetor that uses diaphragms for controlling fuel flow has been filled with fuel and its inlet and vapor vent outlet plugged. How long must the fuel-filled unit be allowed to stand?

  1. AA minimum of 30 minutes
  2. BA minimum of 8 hours
  3. CA minimum of 24 hours
Show the answer and explanation

Correct answer: B. A minimum of 8 hours

  • A — incorrect A brief soak would not restore the diaphragms to their original pliability, so the calibration of the unit could not be relied upon.
  • B — correct The unit is left standing full of fuel for a minimum of 8 hours so the diaphragms soak and become as pliable as when the carburetor was calibrated.
  • C — incorrect Twenty-four hours exceeds the period given in the procedure; the specified minimum soak time is 8 hours.

BACKGROUND

When installing a carburetor that uses diaphragms for controlling fuel flow, the fuel lines are connected and the carburetor is filled with fuel. This is done by turning on the fuel boost pump and moving the mixture control from idle cutoff to rich, continuing the flow until oil-free fuel comes from the drain valve, which shows that the preservative oil has been flushed out. The fuel flow is then turned off, the fuel inlet and vapor vent outlet are plugged, and the carburetor, full of fuel, is allowed to stand for a minimum of 8 hours. This soak is necessary to make the diaphragms pliable to the same degree as when the unit was originally calibrated. Mounting bolts are then torqued to the values in the applicable maintenance manual.

STUDY TIP

Remember the reason behind the wait, not just the number: dry diaphragms are stiff and the unit was calibrated with them soft. Watching for oil-free fuel at the drain is the cue that the preservative oil has been flushed.

KEY POINTS

  • Flush a diaphragm carburetor with fuel until oil-free fuel appears at the drain valve.
  • Plug the fuel inlet and vapor vent outlet and let the fuel-filled unit stand at least 8 hours.
  • The soak restores diaphragm pliability to the condition present at original calibration.

SOURCES

  • amp-powerplant/ch02.txt "Installation of Carburetor"
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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.