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Fuel leaking from a float carburetor discharge nozzle

Aviation Mechanic Powerplant (AMP) practice question 7 of 20 on Engine Fuel and Fuel Metering Systems. ACS element AM.III.I.K2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONFuel is found leaking from the discharge nozzle of a float-type carburetor while the engine is not running. What is the most likely cause?

  1. AA main air bleed that is plugged with carbon
  2. BA leaking float needle valve and seat
  3. CAn idle mixture adjusted too rich
Show the answer and explanation

Correct answer: B. A leaking float needle valve and seat

  • A — incorrect The main air bleed only reduces fuel density and improves vaporization during operation; blocking it cannot raise the fuel level or push fuel out of a stopped engine's nozzle.
  • B — correct The handbook states directly that fuel leaking from the discharge nozzle with the engine stopped most likely means the float needle valve and seat are leaking and must be replaced.
  • C — incorrect An overly rich idle mixture causes spark plug fouling and incomplete combustion while running, but it has no effect on the carburetor when the engine is shut down.

BACKGROUND

A float chamber is provided between the fuel supply and the main metering system and serves as a reservoir for fuel in the carburetor. The chamber holds a nearly constant fuel level about one-eighth inch below the holes in the main discharge nozzle. That level is kept slightly below the nozzle outlet holes both to give the correct fuel flow and to prevent fuel from leaking out of the nozzle when the engine is not operating. The level is controlled by a float-operated needle valve and seat; the float rises as fuel enters and closes the valve at the predetermined level. If the needle valve and seat leak, fuel keeps entering, the level rises above the nozzle holes, and fuel drips out.

STUDY TIP

Tie the symptom to the geometry: the nozzle holes sit above the normal fuel level, so anything that raises that level produces a leak. Remember that the needle valve and seat are the only things holding the level in place.

KEY POINTS

  • A float chamber keeps fuel about one-eighth inch below the main discharge nozzle holes.
  • A float-operated needle valve and seat maintain that level as fuel is drawn out.
  • Fuel dripping from the nozzle with the engine stopped points to a leaking needle valve and seat.

SOURCES

  • amp-powerplant/ch02.txt "Float Chamber Mechanism System"
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MORE ON ENGINE FUEL AND FUEL METERING SYSTEMS

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.