FAA Powerplant Written Practice Test

What occurs when the optimum idle mixture has been achieved while adjusting the mixture control?

1. The engine immediately stops firing

2. Manifold pressure decreases and rpm increases momentarily

3. Fuel flow stabilizes consistently

4. The engine runs at maximum RPM

The correct answer is: Manifold pressure decreases and rpm increases momentarily

When the optimum idle mixture is achieved while adjusting the mixture control, there is a momentary increase in engine RPM along with a decrease in manifold pressure. This phenomenon occurs because the engine is reaching a point where the air-fuel mixture is most favorable for combustion at idle, allowing for more efficient operation. As the mixture becomes more optimal, the engine requires less fuel for the same amount of air, resulting in a leaner mixture that can activate a more efficient combustion process, which in turn increases RPM momentarily. The drop in manifold pressure indicates that the engine is operating more efficiently, with a better air-fuel mix, which leads to improved performance. Although RPM might increase briefly due to the ideal mixture allowing for smoother and more effective combustion, it's essential to note that this adjustment does not mean that the engine is now running at maximum RPM; instead, it's simply responsive to the adjustments made in the mixture control that have brought it to an optimal idle setting.