FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A mechanic is checking the idle mixture on a float-type carburetor. If, after leaning the mixture control, the engine RPM increases significantly (e.g., 50 RPM or more) before decreasing, what does this indicate?
- AThe carburetor float level is too low.
- BThe idle mixture is too lean.
- CThe idle mixture is too rich.
- DThe idle mixture is set correctly.
Show answer & explanationAnswer & explanation
Correct answer: C. The idle mixture is too rich.
During an idle mixture check, a significant RPM rise (50 RPM or more) before a drop indicates that the idle mixture was initially too rich. Leaning the mixture brings it closer to the ideal fuel-air ratio for best power at idle, causing the RPM to increase before it becomes too lean and drops off.
Why the other options are wrong
- A. A too-low float level would generally result in a lean mixture, not a rich one that causes a significant RPM rise when leaned.
- B. If the idle mixture were too lean, leaning it further would cause an immediate drop in RPM, not an increase.
- D. A correctly set idle mixture would show a slight RPM rise (typically 25-50 RPM) before a sharp drop, not a significant increase.
Idle Mixture Check (Carburetor)
A procedure to adjust the idle fuel-air mixture on a float-type carburetor, typically indicated by engine RPM response when leaning the mixture.
- Too rich: significant RPM rise (50+ RPM) when leaning.
- Too lean: immediate RPM drop when leaning.
- Correct: slight RPM rise (25-50 RPM) then drop when leaning.
Memory trick: Rich RPM Rises, Lean RPM Lowers