RCB 9D-5


	Bulletin 9D-5 August, 1951 Model "AA" Page 4 ROCHESTER PRODUCTS DIVISION, GENERAL MOTORS CORP., ROCHESTER, NEW YORK


	IDLE SYSTEM The carburetor idle system controls the fuel/ air mixtures from the car idle setting up to approximately 30 mph. As shown on Figure 2 the fuel first passes through the centrally located main metering jets "A" to the Idle Tubes "B" in the bowl, each of which have calibrated orifices. These orifices control the amount of solid fuel metered by the idle system. At the junction of the cover and bowl air mixes with the solid fuel through two calibrated air bleeds "C". This fuel/air mixture then passes horizontally through a cored passage in the cover; additional air is also metered by the idle channel bleed "D". The mixture continues to follow the Figure 2. Idle System path of suction downward through the idle passages in the Housing to the Throttle Body where it is metered to the engine adjusting screws, and the secondary and top idle holes "E" as they are exposed suction by the opening of the throttle valves "F".	by the idle to manifold

	PART THROTTLE SYSTEM At a point of sufficient throttle opening manifold vacuum or suction is applied to the nozzles, which then take over the fuel delivery to the engine. This point is termed the transfer point as the fuel delivery is transferred from the idle system to the main metering system. To a large degree the calibrated main metering jets govern the fuel mixtures throughout this range of 30-60 mph, assisted by the air bleeds in the Main Well Tubes admitting varying amounts of air from within the outer bowl to mix with fuel passing from the nozzles according to engine demand. As shown on Figure 3 the throttle valves "F" are opened sufficiently so that the manifold suction is now greater upon the nozzles ^Figure ^3. ^Part ^Throttle ^System "G" than at the idle discharge holes "E" in the throttle body; as a consequence, fuel continues to pass through the calibrated main metering jets "A" to the main wells. However, the suction being now greater upon the nozzlcs, the fuel passes upward along the Main Well Tubes "H" rather than through the Idle Tubes and out the discharge nozzles "G". The Main Well Tubes "II" have four calibrated holes which admit air from the main well bleed "C" in the Cover. As the throttle valves are opened to a greater degree the suction is increased upon the nozzles causing them to deliver more fuel to meet the engine demand. This increase in fuel delivery lowers the fuel level in the main wells; consequently, the holes in the Main Well Tubes arc exposed in turn by the lowering of the fuel level to admit more air to