AMT Engine Fuel Metering Systems 3 — Questions and Answers
Question 1: What effect does carburetor ice formation have on engine operation?
- Increases manifold pressure and RPM
- Causes a progressive loss of engine power due to reduced airflow and mixture enrichment (Correct answer)
- Has no effect until the venturi is completely blocked
- Improves atomization of fuel at low temperatures
Correct answer: Causes a progressive loss of engine power due to reduced airflow and mixture enrichment
Carburetor ice forms in the venturi throat, restricting airflow and progressively richening the mixture, causing a loss of power that worsens if untreated.
Question 2: In a fuel injection system, what is the result of a clogged injector nozzle on the affected cylinder?
- The engine will run slightly rich on that cylinder
- That cylinder will run lean, potentially causing overheating or misfiring (Correct answer)
- Fuel will be redistributed to the remaining cylinders with no power loss
- The flow divider will compensate by increasing fuel to adjacent cylinders
Correct answer: That cylinder will run lean, potentially causing overheating or misfiring
A clogged injector restricts fuel to that cylinder, creating a lean condition that can cause overheating, rough running, or detonation.
Question 3: The fuel-air mixture control in a float carburetor changes the mixture by:
- Altering the size of the main metering jet
- Moving a needle or plate that restricts the fuel passage in the main discharge circuit (Correct answer)
- Changing the float level to vary fuel bowl pressure
- Adjusting throttle plate position relative to the venturi
Correct answer: Moving a needle or plate that restricts the fuel passage in the main discharge circuit
The mixture control needle or plate restricts or blocks the passage between the float chamber and the main discharge nozzle to lean or richen the mixture.
Question 4: Which type of carburetor system is LEAST susceptible to carburetor ice because the fuel-air mixing does not occur in the venturi?
- Updraft float-type carburetor
- Downdraft float-type carburetor
- Direct fuel injection system (Correct answer)
- Pressure carburetor with an impact tube
Correct answer: Direct fuel injection system
Direct fuel injection introduces fuel at each cylinder intake port rather than in a venturi, so the cooling and ice-forming conditions present in carburetors do not occur.
Question 5: An automatic mixture control (AMC) in a carburetor compensates for altitude by:
- Increasing venturi size as altitude increases
- Using an aneroid bellows to lean the mixture as air density decreases with altitude (Correct answer)
- Opening a supplemental air valve above a set altitude
- Electronically adjusting the metering jet based on barometric pressure
Correct answer: Using an aneroid bellows to lean the mixture as air density decreases with altitude
The AMC uses an aneroid bellows that expands as ambient pressure decreases, mechanically leaning the mixture to maintain the correct fuel-air ratio with altitude.
Question 6: What would a fuel flow gauge reading significantly HIGHER than normal on a fuel-injected engine most likely indicate?
- A blocked injector nozzle
- A stuck-open flow divider valve (Correct answer)
- A leak in the fuel return line causing false high pressure
- The mixture control is in the full rich position at altitude
Correct answer: A stuck-open flow divider valve
A stuck-open flow divider can cause abnormally high fuel flow indications and rich running because fuel bypasses metering and floods the distribution system.
Question 7: During a ground run-up, a carburetor heat check should produce:
- A significant increase in RPM as the mixture leans out
- An initial drop in RPM, possibly followed by a slight increase if ice was present (Correct answer)
- No change in RPM if the carburetor is functioning correctly
- An increase in manifold pressure and a richer exhaust color
Correct answer: An initial drop in RPM, possibly followed by a slight increase if ice was present
Applying carb heat introduces warmer, less dense air causing an RPM drop; if ice was present, the RPM may recover slightly as the ice melts and airflow improves.
AMT Engine Fuel Metering Systems 3
What effect does carburetor ice formation have on engine operation?