What is the RPM indication when an idle mixture is set correctly and the mixture control is placed to idle cutoff?

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

Answer: (C)

When the idle mixture is set correctly, the engine is tuned to idle smoothly on the idle circuit. Placing the mixture control to idle cutoff removes fuel from that circuit. As the fuel flow is cut, the engine doesn’t die instantly; the transition causes a brief moment where the air–fuel mixture becomes lean enough to burn efficiently for a split second, so the RPM can rise slightly before the fuel is fully cut and the engine slows down. That brief, small uptick in RPM is the expected indicator of a proper idle setup, followed by a decline as fuel is completely shut off.