Carburetor Solenoid: Description and Operation
Fig. 44 Idle enrichment system
The idle enrichment system, Fig. 44, improves cold engine driveability by using a metering system related to the basic carburetor instead of the choke.
During cold engine operation, the complete idle system will be enriched.
Idle system air is controlled by a small diaphragm mounted near the top of the carburetor. When control vacuum is applied to the diaphragm, idle
system air is reduced, resulting in an increased fuel flow and a richer air/fuel mixture.
The vacuum signal to the carburetor diaphragm is controlled by a thermal switch which is in contact with the engine coolant flow. When the engine is
cold, the switch is open and the vacuum signal is passed to the diaphragm. As the engine warms, the switch closes, eliminating the vacuum signal and
returning the carburetor metering to normal, lean levels. On some systems, this engine coolant vacuum switch controls the duration of idle system
In other systems, similar coolant vacuum switches are used, but the switch receives its vacuum signal from a solenoid valve operated by an electric
timer. Idle enrichment will last for approximately 35 seconds. When the engine warms, the thermal switch will prevent additional cycles of idle
enrichment, however, when the engine is cold, each restart will cycle the enrichment for an additional 35 seconds.
The switches used in solenoid controlled systems close at approximately 150° F., while others close at approximately 98° F. All switches open at
approximately 12° F.