Why must a float-type carburetor supply a rich mixture during idle?

A float-type carburetor is designed to supply a rich fuel mixture during idle primarily to ensure that the engine runs smoothly and reliably at this low speed and low airflow condition. At idle, the airflow through the carburetor is minimal, which means there is less air available to mix with fuel. A richer mixture compensates for this by providing more fuel than would be necessary if the engine were operating at higher speeds where airflow is increased.

When the air-fuel mixture is rich during idle, it helps to maintain combustion stability. Since idle conditions can lead to incomplete combustion if the mixture is too lean, a rich mixture helps avoid stalling and ensures that the engine continues to run effectively. This is critical for engine performance and efficiency, as it allows for proper fuel combustion and smoother operation, helping to prevent issues such as engine sputtering or dying.

Moreover, while preventing overheating and providing sufficient engine cooling can be important benefits of ensuring a proper fuel mixture, they are not the primary reasons for needing a rich mixture during idle. The key function at this phase is to maintain adequate engine performance through a rich mixture that supports stable combustion.