1942 - 1947 Chevrolet Shop Manual



	6-35

	the surrounding blanket of air passing into the second venturi and this process is repeated by the air in the main venturi. By this means the fuel mixture is carried to the cylinders in a more perfectly atomized condition. This insulated atomi-zation results in increased smoothness of operation at both low and high speeds. The fuel mixture quality is controlled by a metering rod operating in the metering rod jet, and operated by the throttle lever. There are two steps of different diameters on this metering rod. The larger diameter, or economy step, is tapered and controls the gasoline flow to about seven-eighths throttle, at which time the smaller diameter, or power step, becomes effective, giving full power for either high speed or hard low speed pulling. By this simple means both maximum power and greater economy are available without changing the carburetor adjustment. Fig. 71 is a cross-section view of the carburetor showing the various passages, jets. etc. The carburetor used on all passenger cars and trucks (except cab-over-enginc models) is identical and is known as the downdraft "balanced" type. The air pressure in the carburetor float chamber is balanced with the pressure on the inside of the air horn by a system of passages in the carburetor. The air intake for the balance passage consists of a brass tube pressed into the wall of the air horn and extending to the center of the air horn. This tube connects with a passage in the side of the air horn as shown in Fig. 72. With this balanced pressure, the proportions of air and gasoline in the mixture delivered to the engine remain substantially the same at all times, even when the air cleaner is restricted by dirt. The choke consists of a one-piece choke valve, fastened by the means of two screws to the choke shaft which is offset to one side of the carburetor air horn. The valve is machined with an angle on	each end to permit solid seating against the wall of the air horn. Due to the choke valve seating against the walls of the air horn, it cannot be damaged by backfire. The choke lever is mounted on a boss on the air horn and is retained by a snap ring fitted into a groove in the boss. The choke lever floats on the shaft and is connected to it through a light coil spring. This spring hooks to the choke lever and to the choke shaft. In this way the choke valve is operated by the choke lever through the spring. As the choke button on the instrument panel is pulled out, the light valve spring causes the valve to follow the movement of the lever, closing the choke valve. As the engine starts, the rush of incoming air through the air horn of the carburetor overcomes the tension of this spring and the valve automatically assumes the correct position to provide the proper amount of air for the mixture to enter and prevent over-choking. The accelerating pump consists of a cylinder with a plunger containing an air bell and two check valves, one on the inlet and one on the outlet side. The accelerating pump plunger including the shaft, guide and leather is made as an assembly. The shaft is rectangular and bears in the bowl cover. The inlet check valve has a bakelite disc while the outlet check valve has a brass disc. The brass disc aids fuel economy since its weight tends to keep it on its scat and prevent lifting from the seat due to the air velocity in the carburetor. The upward movement of the pump plunger, when the throttle is closed, draws a small metered quantity of gasoline into the bottom of the cylinder. The slightest opening of the throttle (downward movement of the plunger) causes an immediate discharge through the pump jet pointing downward into the main venturi. This action is illustrated in Fig. 73. Gasoline enters the low speed jet through a 3/64" hole drilled through the jet at the recess between the threaded section and the base of the jet. which coincides with the low speed well in the carburetor body, Fig. 71. The metering hole is drilled vertically and is .035" in diameter. This design prevents the possibility of the engine stalling due to gasoline surging away from this jet when the brakes are applied suddenly. The main nozzle is of two-piece construction with the inner nozzle pressed into the outer nozzle, thereby maintaining the proper relationship between the openings in the walls of the two nozzles. The nozzle assembly is held in place by a brass plug having a 1/8" hole drilled through its center.

	Fig. 72—Carburetor Balance Passages