What does a slight rise in RPM indicate when putting the mixture in the idle-cutoff position?

A slight rise in RPM when placing the mixture in the idle-cutoff position indicates that the engine is receiving a richer mixture than it needs to maintain idle, which contradicts the expected behavior during the transition to idle-cutoff. Specifically, as the mixture is adjusted to idle-cutoff, the fuel flow is reduced, and the RPM should ideally drop due to the decrease in fuel reaching the engine.

If instead, the RPM increases slightly, it suggests that the idle mixture is not excessively rich initially—meaning it is somewhat balanced or correctly set. The slight rise in RPM demonstrates that the engine is still able to combust adequately with the existing fuel mixture even as some fuel is cut off. This implies that the mixture isn't so rich as to overwhelm the engine, but rather it is at a functional point where the engine can still operate effectively before fuel is completely cut off.

The other options would imply issues with the engine or mixture settings that wouldn't align with this behavior, such as being too rich or dealing with mechanical failures evident in other symptoms.