What is indicated by a correct idling mixture when the mixture control is pulled toward the IDLE CUTOFF position?

When the mixture control is pulled toward the IDLE CUTOFF position, a correct idling mixture is indicated by an increase in RPM, followed by a decrease. Specifically, an increase of 10 to 50 RPM before the engine starts to decrease in speed shows that the mixture is correct for optimal combustion at idle.

This increase happens because the engine responds to a slightly leaner mixture, which helps combustion efficiency at idle. The subsequent decrease occurs because, as the mixture becomes excessively lean, it can no longer sustain proper combustion, leading to a reduction in RPM. This behavior provides a useful indicator of the engine’s fuel-air mixture balance at idle, and recognizing it is key to ensuring the engine runs smoothly and efficiently.

The other choices reflect different RPM behaviors that do not accurately indicate a correct idling mixture as described in the question.