See Figures 1 and 2Gasoline Engines
This system is designed to reduce HC emissions and also to improve fuel economy during deceleration.
The system is operated by an anti-dieseling solenoid valve in the carburetor which is controlled by a vacuum switch. When the intake manifold vacuum increases to an extremely high level (which it does during deceleration), the fuel flow of the slow system is shut off by the anti-dieseling solenoid valve. When the intake manifold vacuum drops to a low level again, the fuel flow to the slow system is resupplied.
The fuel shut-off system is further controlled by the clutch switch and gear position switches such as the neutral switch (manual transmission) and the inhibitor switch (automatic transmission) to ensure that fuel cannot be shut off even if the manifold vacuum is high enough to trigger the normal fuel shut-off operation.