For maximum endurance the mixture control should be set to:

For maximum endurance in reciprocating engines, the mixture control should be set to a weak or lean mixture. This setting is important because a weak mixture allows for optimal fuel efficiency, which is crucial for endurance flying or long-duration operations. In this state, the engine operates with less fuel relative to the amount of air, leading to a reduction in fuel consumption while still maintaining sufficient power and performance.

Operating with a weak mixture results in a lower fuel flow, which extends the range of the aircraft or engine by conserving fuel. This is particularly advantageous in situations where fuel economy is prioritized over maximum power, allowing for longer flight times.

On the other hand, richer mixtures typically result in more power and a smoother operation but at the cost of increased fuel consumption, which is not ideal when endurance is the primary concern. Therefore, choosing a weak mixture strikes the best balance, allowing the engine to run efficiently over extended periods.