What impact does altitude have on fuel metering in an engine without automatic mixture control?

Altitude has a significant impact on the air density surrounding an engine, which directly affects the air-fuel mixture. As altitude increases, the atmospheric pressure decreases, resulting in less air density. In an engine without automatic mixture control, this reduced air density means that fewer air molecules are available for combustion.

A lean mixture occurs because the fuel system is delivering a fixed amount of fuel regardless of the surrounding conditions. With less air available at higher altitudes, the fuel-to-air ratio becomes unbalanced, leading to a lean mixture formation. This condition can result in less efficient combustion and may require adjustments to the fuel system in order to maintain optimal engine performance under varying altitude conditions.

In contrast to the other options, a richening of the mixture would typically occur at lower altitudes where more air is available, while a stable mixture ratio would not be expected without automatic mixture control to adapt to changing altitudes. Increased fuel efficiency is not guaranteed at higher altitudes since running too lean can lead to engine issues and decreased performance.