What occurs to the mixture when ascending to altitude on a carburetor without automatic mixture control?

In a carburetor system without automatic mixture control, as altitude increases, the atmospheric pressure decreases. This reduction in pressure affects the density of the air entering the carburetor. Since the carburetor relies on the intake air to mix with fuel for combustion, a lower air density results in a larger proportion of fuel relative to the thinner air, leading to a richer mixture.

A richer mixture implies that there is more fuel available in relation to the air, which can cause issues such as engine roughness or potential fouling of spark plugs if the aircraft does not adjust for the decreased airflow. Without automatic mixture adjustment, pilots must manually lean the mixture when flying at higher altitudes to compensate for this change, ensuring optimal performance and avoiding excessive fuel consumption or engine roughness.

In contrast to the correct understanding, the other options either suggest that the mixture remains constant or becomes leaner, which is not accurate without manual intervention.