FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A mechanic is checking the idle mixture on a float-type carburetor. If the engine RPM increases significantly and then drops when the mixture control is moved to idle cutoff, what does this indicate?
- AThe idle RPM is set too high.
- BThe idle mixture is too lean.
- CThe carburetor float level is set too low.
- DThe idle mixture is too rich.
Show answer & explanationAnswer & explanation
Correct answer: D. The idle mixture is too rich.
When the mixture control is moved to idle cutoff, a properly set rich idle mixture will cause a slight momentary increase in RPM (10-50 RPM) as the mixture leans out to chemically correct before fuel flow ceases. A significant increase indicates an excessively rich idle mixture.
Why the other options are wrong
- A. High idle RPM might mask mixture issues but doesn't directly cause this specific RPM increase pattern.
- B. If the idle mixture is too lean, there would be little to no RPM increase, or the engine might stumble and die immediately.
- C. A low float level would typically lead to a lean condition, which is the opposite of what this symptom indicates.
Idle Mixture Check (Carburetor)
A procedure to adjust the fuel-air ratio at idle by observing engine RPM changes when the mixture control is moved to idle cutoff.
- Slight RPM rise (10-50 RPM) indicates correct mixture.
- Significant rise indicates rich mixture.
- No rise or immediate drop indicates lean mixture.
Memory trick: Big RPM jump at cutoff? Too much gas, engine was 'gassy'.