Idle Circuit

The fuel delivery in a carburetor tends to lag behind the motion of the throttle. The basic carburetor operates when the throttle valve is fully open or partially open, but not when it's closed. No driver wants the engine to stop every time the foot leaves the accelerator; such a car would be tiring and stressful to drive, even in the best of road conditions, let alone in a traffic situation. To keep the engine running smoothly and evenly when no power is needed, the idle circuit was added inside the carburetor. The idle jet admits fuel on the engine side of the throttle valve. Additional air is mixed with this fuel through an air bleed. The result is an entirely separate carburetor circuit which operates only when the throttle valve is closed.