FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationEasy
During a ground run, a mechanic performs an idle mixture check on a float-type carburetor by slowly moving the mixture control toward idle cutoff while watching the tachometer. The engine RPM begins to decrease immediately with no momentary rise before it drops. What does this indicate?
- AThe idle mixture is set too rich
- BThe carburetor float level is too high
- CThe idle mixture is set too lean
- DThe idle mixture is correctly adjusted
Show answer & explanationAnswer & explanation
Correct answer: C. The idle mixture is set too lean
A correctly adjusted idle mixture will show a momentary RPM rise of about 25-50 RPM before the engine dies when leaned to cutoff, because the mixture briefly passes through a more efficient ratio. If RPM drops immediately with no rise, the mixture is already too lean at idle, so leaning further only worsens combustion.
Why the other options are wrong
- A. Too rich would produce a larger-than-normal RPM rise before dying.
- B. Float level primarily affects fuel metering volume, not this specific idle cutoff test result.
- D. A correct setting always shows the characteristic momentary rise.
Idle Mixture Check
A ground test where the mixture control is moved toward idle cutoff to verify idle fuel/air ratio by observing tachometer response.
- Correct setting: momentary 25-50 RPM rise before engine dies
- No rise/immediate drop = too lean
- Rise greater than spec = too rich
- Return mixture to rich promptly after the check
Memory trick: 'Rise then die' means it's set just right.