The 4GC models use an automatic choke while the 4G models use a manual choke.
There are three designs of automatic choke systems used on the 4GC models. We will refer to them as the convetional system, the split linkage system, and the hot water system.
The conventional choke system may have the choke housing assembly mounted on the air horn, float bowl, or throttle body. On units with the housing on the bowl or throttl ebody, an intermediate choke rod adjustment is necessary. The principles of operation are the same on all units.
Choking of the carburetor is necessary only on the primary side as the secondary throttle valves are locked in the closed position whenever the choke valve is partially closed. This is accomplished by a secondary throttle shaft lock out lever and a slot in the fast idle cam. Whenever the choke valve is closed, the lock out lever prevents opening of the secondary throttle valves. When the choke vavle is wide open, the fast idle cam drops down so that the lock out lever clears the cam, permitting the secondary throttle valves to open.
The choke system consists of a thermostat coil aseembly, choke piston, off set choke valve and fast idle cam and linkage. Its operation is controlled by a combination of intake manifold vacuum, the off set choke valve, and temperature.
When the engine is cold, the thermostatic choil is calibrated to hold the choke vavle closed. As the engine is started, air velocity against the off set choke valve causes it to open slightly against the torque of the thermostatic coil. In addition, intake manifold is applied to the choke piston through a vacuum passage which also tends to open the choke valve. The choke valve assumes a position where the torque of the thermostatic coil is balanced agianst vacuum pull on the choke piston and air velocity against the off set choke valve. This results in a regulated air flow into the carburetor which provides a richer mixture during the warm-up pweriod.
During the warm-up period the vacuum piston serves to modify the choke action to compensate for varying engine loads or acceleration. Any acceleration or increased load decreases the vacuum pull on the choke piston. This allows the thermostatic coil to momentarily increase choke valve closure to provide the engine with a richer mixture for acceleration.
As the engine warms up, hot air from a tube heated by exhaust gas is drawn into the thermostatic coil housing. The hot air causes the coil to slowly relax its tension. Thus the choke valve is allowed to move gradually to the full open position.
To prevent stalling during the warm-up period, it is necessary to run the engine at a slightly higher idle speed than for a warm engine. This is accomplished by the fast idle screw which rests on the steps of the fast idle cam. The fast idle cam is linked to the choke valve shaft by the choke rod, choke trip lever and choke lever and collar assembly. This holds the throttle valves open sufficiently during the warm up period to increase the idle RPM until the choke valve moves to the full open position.
When the automatic choke is in operation the driver may wich to advance the throttle to the full wide open position. Since this would decrease the pull upon the choke piston thereby closing the choke valve, it is necessary to provide increased carburetor air flow by opening the choke valve mechanially. To accomplish this, a tang ont he fast idle cam is made to contact the throttle lever at wide open throttle position to sufficiently open the choke valve. This is called a choke unloader and also serves to de-choke a flooded engine during starting, whenever the engine is started with the accelerator held fully depressed.
The split linkage choke is designed to let the choke valve and fast idle cam work independently. The operation of the coil and piston is the same as the conventional system. The split linkage operates in the following manner.
The intermediate choke rod is attached to a hole in the end of the intermediate choke lever, while the choke rod is attached to a hole half-way out on the lever. The hole for the intermediate choke rod is at a greater distance from the pivot point than the choke rod. The result is, that as the thermostatic coil warms up and allows the weight of the intermediate choke lever to rotate the lever clockwise, the intermediate choke rod will drop faster than the choke rod and allow the choke valve to open while still maintaining a fast idle.
With this design, it provides provides a relatively short choking period with adequate fast idle for a cold engine.