RCB 9D-9 Model "4GC"


	ROCHESTER CARBURETORS Bulletin 9D-9 January 1952
	Page 10 ROCHESTER PRODUCTS, DIVISION OF GENERAL MOTORS, ROCHESTER, NEW YORK

	CHOKE SYSTEM




	Figure 7-6

	Use Figure 7-6 as a reference. The Model 4GC Carburetor employs the use of a fully automatic choke to insure proper starling and driving during cold weather operation. Choking of the carburetor is necessary only on the primary or pump side. This is due to the fact that the secondary throttle valves are locked in the closed position whenever the choke valve is even partially closed. This is accomplished by a secondary throttle shaft lock out lever (1) and a slot in the fast idle cam (2). Whenever the choke valve is closed the lock out lever prevents opening of the secondary throttle valves. However, when the choke valve is wide open, the fast idle cam drops down so that the lock out lever clears the cam, thus permitting the secondary throttle valves to open. The choke system is composed of a thermostatic coil (3), vacuum piston (4), offset choke valve, and fast idle cam (2). Its operation is controlled by a combination of intake manifold vacuum the offset choke valve atmospheric temperature, and exhaust manifold heat. When the engine is cold the thermostatic coil is calibrated to hold the choke valve closed. As the engine is started, air velocity against the offset choke valve causes the valve to open slightly against the torque of the thermostatic coil. In addition, intake manifold vacuum is applied to the vacuum piston (4), through the vacuum channel (5) which also tends to open the choke valve. Therefore, the choke valve assumes a position where the torque of the thermostatic coil is balanced against vacuum pull upon the choke piston and air velocity against the offset choke valve thereby causing a regulated air flow into the carburetor which provides a richer mixture during the warm-up period. During warm-up the Vacuum piston (4) serves to modify the choking action to compensate for varying engine loads or acceleration. Any acceleration or in-		creased road load decreases the vacuum exerted on the choke piston. This allows the thermostatic coil torque to momentarily increase choke valve closure to provide the engine with a richer mixture for acceleration. As the engine warms up, hot air from the exhaust manifold is drawn into the thermostatic coil housing through (6). This hot air causes a rise in temperature which in turn 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 (7) which rests on the steps of the fast idle cam (2). The fast idle cam is in turn linked to the choke valve shaft (8) by the choke rod (9), choke trip lever (10) and choke lever and collar assembly (11) and holds the throttle valves open sufficiently during the warm-up period to give the increased idle RPM, until the choke valve moves to the full open position. While the automatic choke is in operation the driver may wish to advance the throttle to the full wide open position. Since this would decrease pull upon the vacuum piston (4) thereby closing the choke valve, it is necessary to provide increased carburetor air flow by opening the choke valve mechanically. 'To accomplish this, a tang (1 2) on the fast idle cam (2) is made to contact the throttle lever (13) at wide open throttle position so as to sufficiently open the choke valve. This is also called a choke unloader and serves to dechole a flooded carburetor during cold starting operation. This choke unloader will also relieve a Hooded condition on starting by allowing more air to enter the carburetor and mix with the excess gasoline in the manifold whenever the engine is cranked with the accelerator held fully depressed.