AMT Engine Fuel Metering Systems 2 — Questions and Answers
Question 1: What is the primary function of an accelerator pump in a float-type carburetor?
- Increase idle fuel flow during warm-up
- Provide extra fuel during rapid throttle opening to prevent a lean mixture (Correct answer)
- Reduce fuel pressure during high-power settings
- Meter fuel during cruise flight conditions
Correct answer: Provide extra fuel during rapid throttle opening to prevent a lean mixture
The accelerator pump delivers an extra shot of fuel when the throttle is opened quickly, preventing a momentary lean mixture that could cause engine stumble.
Question 2: In a pressure-injected carburetor, the fuel control unit meters fuel based primarily on which parameter?
- Throttle position only
- Manifold absolute pressure
- The differential pressure between impact air and venturi suction (Correct answer)
- Engine RPM as sensed by a governor
Correct answer: The differential pressure between impact air and venturi suction
Pressure-injected carburetors use the differential between impact (ram) air pressure and venturi suction to meter fuel flow proportional to airflow.
Question 3: Which component in a continuous-flow fuel injection system controls fuel distribution to individual cylinders?
- Fuel control unit
- Flow divider (fuel manifold valve) (Correct answer)
- Engine-driven fuel pump
- Mixture control arm
Correct answer: Flow divider (fuel manifold valve)
The flow divider (manifold valve) receives metered fuel from the fuel control unit and distributes it equally to each cylinder's injector nozzle.
Question 4: What is the purpose of the idle cut-off position on a mixture control?
- Enrich the mixture for starting
- Stop fuel flow to shut down the engine cleanly (Correct answer)
- Activate the primer system
- Switch from main tanks to auxiliary tanks
Correct answer: Stop fuel flow to shut down the engine cleanly
Moving the mixture control to idle cut-off stops fuel flow to the engine, allowing shutdown by fuel starvation rather than ignition cutoff, which burns residual fuel and prevents after-firing.
Question 5: A carburetor's economizer system (power enrichment valve) is designed to:
- Lean the mixture at cruise power to improve fuel economy
- Enrich the mixture at high power settings to prevent detonation and overheating (Correct answer)
- Provide additional fuel during engine starting
- Compensate for altitude changes automatically
Correct answer: Enrich the mixture at high power settings to prevent detonation and overheating
The economizer (power enrichment valve) opens at high power settings to richen the mixture, providing cooling and preventing detonation.
Question 6: In an RSA fuel injection system, the fuel control unit is driven by:
- An electric motor independent of the engine
- Engine oil pressure
- Airflow through the throttle body, sensed by an air diaphragm (Correct answer)
- A mechanical connection to the magneto
Correct answer: Airflow through the throttle body, sensed by an air diaphragm
The RSA injector's air diaphragm senses airflow by measuring the pressure differential across the throttle body and uses it to position the metering valve.
Question 7: When troubleshooting a float-type carburetor that floods excessively, the FIRST item to inspect is:
- Accelerator pump check valve
- Float level and needle valve for wear or contamination (Correct answer)
- Main discharge nozzle for blockage
- Idle mixture adjustment screw
Correct answer: Float level and needle valve for wear or contamination
Flooding is most commonly caused by a float set too high, a leaking float, or a contaminated/worn needle valve that fails to seat properly.
AMT Engine Fuel Metering Systems 2
What is the primary function of an accelerator pump in a float-type carburetor?