How should the mixture be adjusted if there is no EGT gauge available?

Adjusting the mixture based on engine performance is crucial for optimal engine operation, especially when there is no Exhaust Gas Temperature (EGT) gauge available. The recommended method of leaning the mixture until the engine runs rough and then enriching it until it runs smoothly is a practical and effective approach. This technique allows pilots to find the best fuel-air ratio for the current operating conditions, ensuring efficient combustion and engine performance.

When the mixture is leaned too much, the engine may start to run roughly due to a lack of fuel, which indicates it's not receiving the optimal fuel mixture. By then enriching the mixture until the engine runs smoothly, the pilot can achieve a balanced fuel-air mix that enhances performance without the reference of an EGT gauge. This method also helps prevent potential engine issues that could arise from running too lean.

The other choices may not provide the same level of precision needed for adjusting the mixture. Keeping it set to rich may lead to unnecessary fuel consumption and reduced engine efficiency. Setting it at a predetermined level based on altitude could be overly simplistic and might not account for fluctuating conditions during flight. Similarly, adjusting according to temperature lacks the immediate feedback provided by engine performance, which is vital for making real-time mixture adjustments.