RCB 9D-7 Model "BB"

Bulletin 9D-7 August, 1951 Model "BB" Page 7

ROCHESTER PRODUCTS, DIVISION OF GENERAL MOTORS, ROCHESTER, NEW YORK

CHOKE SYSTEM The model "BB" carburetor employs a fully automatic choke to insure proper starting and driving during cold weather operation. This choke system is composed of a thermostatic coil, piston, choke valve, and fast idle cam linkage. It is controlled by a combination of intake manifold vacuum, the offset choke valve, atmospheric temperature, and exhaust manifold heat. (Figure 7) When the engine is cold the thermostatic coil is calibrated to hold the choke valve closed.
	MODEL "BB" CHOKE SYSTEM Figure 7

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 choke piston which also tends to pull the choke valve open. As a consequence 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 Choke piston serves to modify the choking action to compensate for varying engine loads or acceleration. Any acceleration or increased 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 sufficiently richer mixture for acceleration. As the engine warms up, hot air from the exhaust manifold "stove" is drawn into the thermostatic coil housing. This hot air causes a rise in temperature which causes the coil to slowly relax its tension. Thus the choke valve is allowed to move gradually to the full open position as the engine warms up. 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 cam which is linked to the choke valve shaft and holds the throttle valve open sufficiently during the warm-up period to give the increased idle RPM, until such time as 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 vacuum pull upon the choke piston thereby closing the choke valve, it is necessary to provide increased carburetor air flow by opening the choke valve mechanically.