What is the fundamental reason for adjusting the fuel/air mixture at high altitudes?

The fundamental reason for adjusting the fuel/air mixture at high altitudes is related to the decrease in air density as altitude increases. As an aircraft climbs to higher elevations, the air becomes thinner, meaning there is less oxygen available in a given volume of air. Since internal combustion engines require a specific ratio of fuel to air for optimal combustion, it becomes necessary to reduce the amount of fuel injected into the engine to ensure that the fuel/air mixture remains balanced, allowing for effective engine performance.

If this adjustment is not made, the mixture would become too rich (too much fuel relative to the amount of air), leading to inefficient combustion, potential engine roughness, and decreased power output. Therefore, to maintain proper engine function at high altitudes, the fuel flow is adjusted downward in order to correspond with the reduced availability of air, ensuring a more appropriate fuel/air ratio for combustion.

Balancing the engine temperature and improving fuel combustion are secondary effects of ensuring the fuel/air mixture is correct, but the primary adjustment is driven by the need to accommodate the reduced air density at higher altitudes.