Weakening the mixture below the best fuel/air ratio will cause the engine power to:

Weakening the mixture below the best fuel/air ratio typically leads to a decrease in engine power. This happens because a leaner mixture, which has less fuel relative to air, can cause incomplete combustion. As a result, there is not enough fuel to effectively produce the desired power output. When the mixture is too weak, the engine may not operate efficiently, leading to a loss of power.

At a certain point, the mixture becomes too lean, resulting in rough engine operation, poor fuel economy, and a drop in performance, which is undesirable in engine operation, especially during critical phases like takeoff where maximum power is required. Therefore, maintaining an appropriate fuel/air ratio is crucial for optimal engine performance and power output.