FAA Powerplant Written Practice Test

What indicates that the optimum idle mixture has been reached during engine operation?

1. RPM decreases immediately and the engine continues to run

2. Manifold pressure remains stable without fluctuations

3. Manifold pressure decreases and RPM increases momentarily before the engine ceases to fire

4. Engine runs smoothly without any changes in RPM

The correct answer is: Manifold pressure decreases and RPM increases momentarily before the engine ceases to fire

When evaluating the point at which the optimum idle mixture has been achieved during engine operation, observing a decrease in manifold pressure combined with a momentary increase in RPM clearly indicates that the engine is responding positively to the mixture adjustment. This behavior suggests that the engine is receiving an optimal amount of fuel-air mixture—enough to increase efficiency and power without overwhelming the combustion process. As the mixture is leaned out to reach the optimum point, the decrease in manifold pressure signifies that the engine is getting just the right amount of fuel to maintain efficient combustion. The momentary RPM increase occurs because the engine is becoming more efficient at this adjustment, allowing it to operate at a slightly higher power output for a brief moment. However, if the mixture continues beyond the optimum setting, the engine may eventually falter, leading to a cessation of firing, which is a clear sign that the mixture is too lean for stable operation. In contrast, other indicators such as stable manifold pressure without fluctuations or a smooth engine run without changes in RPM do not specifically confirm that the engine has reached the optimum idle mixture. These signs may represent various operating conditions but don’t provide the clear and defined feedback that adjusting the mixture to its ideal setting provides. Thus, monitoring the precise behavior of RPM and manifold