What does a closed choke do to the engine's air-fuel mixture during starting?

When the choke is closed, it restricts the amount of air that can enter the engine, thereby enriching the air-fuel mixture. This is particularly important during starting, especially in cold conditions, because a richer mixture (more fuel relative to air) helps facilitate ignition and ensures that the engine starts more easily. The closed choke increases fuel delivery compared to air, providing the engine with the necessary resources to catch and run consistently during its initial startup phase.

In practice, closing the choke is a common procedure in small engines when they are cold or when taking off after a period of inactivity, as it compensates for the poorer vaporization of fuel in lower temperatures and improves starting performance.