When is the idle and low-speed circuit in a carburetor inoperative?

The idle and low-speed circuit in a carburetor becomes inoperative when the engine is operating at higher speeds, which typically occurs under conditions such as driving over 20 mph. At these higher speeds, the airflow through the carburetor increases significantly, allowing the main jet to take over fuel delivery to the engine instead of relying on the idle and low-speed circuits.

In other scenarios, like a cold engine, during high acceleration, or while the vehicle is stationary, the idle and low-speed circuits can still function as they are designed to provide the necessary fuel mixture for starting and maintaining engine operation at lower speeds and idle conditions. Hence, the characteristics of carburetor operation reveal that the transition to primary fuel delivery modes occurs predominantly when higher speed thresholds are exceeded.