1942 - 1947 Chevrolet Shop Manual



	*6-37*

	Low Speed At low engine speed the throttle is partly open and suction from the down stroke of the engine piston draws air in through the air horn. The air in passing through the main venturi increases in velocity with the result that the suction is increased over the secondary venturi. This increased air speed through the secondary venturi in turn steps up the suction on the primary venturi and thereby increases the air speed in the primary venturi. The air, passing through the primary venturi, draws gasoline from the main nozzle where it is mixed with the air passing through the primary, secondary and main venturi forming a finely atomized mixture which then passes on to the manifold and cylinders. Fig. 76.	There are two adjustments on the carburetor, one for idling mixture, "A" in Fig. 77, and the other for idling speed, "B" in Fig. 77. Both of these adjustments should be made together. To adjust the idling mixture, proceed as follows: Open the idle adjusting screw "A" from1/2to 1 turn. Allow the engine to idle. Try turning the screw both ways from this position until the best setting is reached.


		Fig. 77—Carburetor Idle Adjustment To adjust for idling speeds, proceed as follows: With the hand throttle on the instrument panel closed, set the throttle lever stop screw "B" so the engine runs at approximately 450 to 500 revolutions per minute. If the engine runs too fast, back the screw out. If too slow, turn it in until the proper speed is obtained. Float Level A 1/2" float level should be maintained for best economy. This measurement should be taken at the end of the float opposite the gasoline intake needle, measured from the top of the float to the machined underside surface of the bowl cover with the gasket removed. The 1/2" gauge shown in Fig. 78 is a convenient tool for making this check. If the float level must be reset, it should be done by bending the lip contacting the gasoline intake needle. Do not make this adjustment by pressing on the float. Bending the lip up will lower the float level and bending it down will raise it. Only a slight bend is necessary to change the float level.
	Fig. 76—Carburetor Low Speed Operation High Speed The carburetor operation at higher speeds is similar to the low speed operation with the exception of raising the metering rod in the metering rod jet. This serves the same purpose as increasing the size of the jet and thereby furnishes the additional gasoline required for high speeds and wide-open throttle power operation. CARBURETOR ADJUSTMENT All carburetors are carefully tested and adjusted to the engine, before leaving the factory. Too often adjustments are made on the carburetor, when in reality, something else is causing uneven running, or the engine has not thoroughly warmed up.