What should happen to engine RPM when moving the mixture control to the idle cutoff position on a normally operating engine?

When the mixture control is moved to the idle cutoff position on a normally operating engine, the expected behavior is that the engine RPM will slightly increase before it starts to die. This occurs due to a momentary lean-out of the fuel-air mixture, which can lead to a brief surge in RPM as the engine adjusts to the rapidly changing mixture.

In typical operation, when the mixture is enriched, the engine runs more efficiently and produces more power, resulting in a higher RPM. Conversely, when moving to idle cutoff, the engine receives a significantly reduced amount of fuel, leading to an increase in RPM as the engine attempts to compensate for the less favorable fuel mixture. However, as the mixture becomes too lean and eventually cuts off fuel, the RPM will then begin to decrease sharply, leading the engine to stall.

This behavior underscores the importance of understanding the fuel system dynamics in piston engines, as it directly informs the pilot about the engine's response to changes in mixture settings.