Which indicator would suggest a weak mixture?

A drop in engine speed serves as an indicator of a weak mixture because it suggests that the fuel-air ratio is not optimal for combustion. When the mixture is weak, typically there is not enough fuel in relation to air to produce the ideal amount of power. This can lead to reduced engine performance and a decrease in power output, causing the engine speed to drop.

In contrast, other options may highlight different issues. For example, white smoke usually points to a coolant leak or excess fuel being burned, while black smoke indicates a rich mixture, where there is too much fuel compared to air. An increase in engine speed accompanied by black smoke suggests an overly rich mixture, leading to inefficient combustion and possible engine performance problems. Hence, the observation of a drop in engine speed is a crucial sign of a weak mixture that may require adjustments for optimum performance and efficiency.