FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician is performing an idle mixture check on a float-type carburetor per the manufacturer's maintenance manual. Upon slowly retarding the mixture control toward idle cutoff at idle RPM, the engine RPM briefly rises 25 to 50 RPM before dying. What does this rise in RPM indicate?
- AThe magnetos are not properly timed
- BThe idle mixture is too lean
- CThe idle mixture is too rich
- DThe idle mixture is correctly adjusted
Show answer & explanationAnswer & explanation
Correct answer: D. The idle mixture is correctly adjusted
A momentary RPM rise of 25 to 50 RPM during an idle cutoff mixture check is the normal, expected indication that the idle mixture is correctly set. It results from a brief improvement in combustion efficiency as excess fuel is removed just before the fuel supply is fully cut off.
Why the other options are wrong
- A. Magneto timing is checked during a magneto drop check at runup, not during an idle mixture check.
- B. A lean mixture typically shows an immediate RPM drop with no rise, or engine roughness before cutoff.
- C. A rich mixture typically shows a larger RPM rise (over 50 RPM) or no immediate drop.
Idle Mixture Check
A procedure where the mixture control is slowly moved toward idle cutoff at idle RPM to verify correct idle fuel/air ratio, indicated by a brief 25-50 RPM rise before the engine stops.
- Performed at idle RPM with mixture slowly leaned to cutoff
- Normal rise is 25-50 RPM (per typical manufacturer specs)
- No rise or immediate drop indicates too lean; excessive rise indicates too rich
Memory trick: A small RPM BLIP before death means the mixture was just right.