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Chamber D pressure in a pressure injection carburetor

Aviation Mechanic Powerplant (AMP) practice question 8 of 20 on Engine Fuel and Fuel Metering Systems. ACS element AM.III.I.K3 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a pressure injection carburetor regulator unit, metered fuel pressure in chamber C is held constant at 5 psi and the regulator maintains a one-quarter psi drop across the metering jets. What is the unmetered fuel pressure in chamber D under this balanced condition?

  1. A4 3/4 psi, one quarter psi below chamber C
  2. B10 psi
  3. C5 1/4 psi
Show the answer and explanation

Correct answer: C. 5 1/4 psi

  • A — incorrect Fuel flows from the unmetered side to the metered side, so chamber D must be higher than chamber C, not lower; a lower pressure would reverse the flow.
  • B — incorrect Ten psi is the constant chamber C value used on some installations, not the chamber D pressure that results from adding the one-quarter psi differential.
  • C — correct Chamber D holds unmetered fuel upstream of the jets, so it must sit one-quarter psi above the 5 psi metered pressure in chamber C to produce the stated pressure drop.

BACKGROUND

The regulator of a pressure injection carburetor is a diaphragm-controlled unit divided into five chambers containing two regulating diaphragms and a poppet valve assembly. Chamber A carries regulated air inlet pressure, chamber B boost venturi pressure, chamber C metered fuel pressure, chamber D unmetered fuel pressure, and chamber E fuel pump pressure. The purpose of the regulator is to hold the fuel pressure at the inlet side of the metering jets according to the mass airflow to the engine. With chamber C held at 5 psi by the discharge nozzle or fuel feed valve, the diaphragm assembly opens the poppet valve until chamber D reaches 5 1/4 psi, giving the balanced one-quarter psi drop across the jets. If nozzle pressure rises, the assembly repositions the poppet valve to re-establish the same differential.

STUDY TIP

Always work the chambers in flow order: pump pressure, unmetered fuel, metering jets, metered fuel. The regulator's whole job is to keep a fixed differential across the jets, so unmetered pressure is always metered pressure plus the differential.

KEY POINTS

  • Chamber C is metered fuel pressure and chamber D is unmetered fuel pressure.
  • The regulator holds a one-quarter psi drop across the metering jets at all airflows.
  • With chamber C at 5 psi, the balanced chamber D pressure is 5 1/4 psi.

SOURCES

  • amp-powerplant/ch02.txt "Regulator Unit"
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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.