What indicates that the optimum idle mixture has been obtained when the mixture control is moved to idle cutoff?

The indication that the optimum idle mixture has been obtained when the mixture control is moved to idle cutoff is signaled by a momentary increase in RPM, followed by a decrease, along with a drop in manifold pressure. This occurs because when the mixture is set to idle cutoff, the engine momentarily runs lean before it ceases to fire entirely due to the lack of fuel.

In a correctly tuned engine, moving the mixture control to idle cutoff leads to a brief lean condition, which might cause the engine to rev slightly (increase RPM) as it gets rid of excess fuel before it ultimately runs out of combustibles and stops firing. The decrease in manifold pressure corresponds with this transition, as there is less airflow (due to reduced fuel) leading towards engine combustion. This brief spike in RPM suggests that the engine was correctly tuned for that moment, providing a clear indicator that the mixture was indeed at an optimal level just before cutting off completely.

The other choices don't accurately capture this mixture management behavior or the resulting engine performance under the idle cutoff adjustment. For instance, stable RPMs or a gradual decline in engine performance without significant changes do not effectively indicate that the optimum mixture has been reached. The temporary change in power and pressure provides a clearer perspective on the