What is observed when idle mixture is correctly set and the idle cutoff is engaged?

• A. A rapid spike in RPM.
• B. No RPM change.
• C. A slight rise in RPM prior to decreasing.
• D. A sudden drop in RPM.

Answer: (C)

When idle mixture is set correctly, the engine runs smoothly at idle and the idle cutoff works as a controlled transition to stop fuel when the throttle is closed. As the idle cutoff starts to take effect, there’s a brief moment where the fuel flow is still adjusting while air continues to flow, so the engine can respond with a small, momentary increase in speed before the fuel is fully reduced. After that transient, the cutoff reduces the fuel enough that RPM falls. This slight rise before the decrease is the normal transient you’d expect with a properly aligned idle system. A rapid spike, no change, or an immediate drop would indicate an abnormal transition or a misadjustment in the idle circuit or cutoff mechanism.