Section 6 - Engine

Idling Adjustment
Adjust the carburetor idle and throttle stop screws in combination with each other to secure the best idling performance. Engine idling speed should be set at 450 to 500 R.P.M.
Cooling System
Tighten all cooling system hose connections and examine for any indications of water leaks. Check the fan belt for proper tension and adjust it if necessary. Proper tension is 1-1/2" total play up and down at a point between the water pump and generator pulleys.
Current and Voltage Regulator
Check the adjustment of the current and volt­age regulator according to the instructions given in Section 12 of this Manual.
Road Test
After completion of the above operations, the car or truck should be road-tested for perform­ance. During this test the octane selector should be adjusted for the grade of gasoline being used. For peak performance and maximum gasoline economy, the octane selector should be set to pro-
duce a slight engine "ping" on acceleration at wide-open throttle.
The downdraft carburetor on Chevrolet cars and conventional trucks contributes to the smooth, quiet operation and power of the Chevrolet valve-in-head engine. Downdraft, as the name implies, means that the gas mixture is drawn down into the intake manifold rather than up from the carbu­retor into the intake manifold as is done in the up-draft carburetor.
The downdraft embodies a principle which em-ploys three Venturis, one located above and two below the level of the gasoline in the float cham­ber. This triple venturi has the effect of increasing the suction on the first or primary venturi, causing the nozzle to start delivering gasoline at very low air speeds. The nozzle enters the primary venturi at an angle, discharging upward against the air stream. This angle provides an even flow of cor­rectly proportioned and finely atomized fuel.
The gasoline thus atomized in the primary ven­turi is kept centrally located in the air stream by
Fig. 71óDiagrammatic View of Downdraft Carburetor





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