RCB 9D-7 Model "BB"

Bulletin 9D-7 August, 1951 Model "BB" Page 5

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

The idle fuel is then metered to the engine by the idle adjusting needles located below the throttle valves. As the throttle valves are opened, the idle discharge holes above the throttle valves are exposed to manifold vacuum, thus becoming fuel discharge holes to meet the increased demand of the engine

PART THROTTLE SYSTEM As the throttle valves arc opened to a greater degree, (Figure 3) the suction created at the main discharge nozzles by means of manifold vacuum and the multiplication created by means of the primary and secondary Venturi, becomes great enough to pull fuel from the nozzles, through the main metering jets, the main well channels and the crossbar channels. At this point air is bled into this fuel through the crossbar bleeds. The fuel is thus discharged from the main dis

charge nozzles rather than through the idle system. The calibration of the Main Metering Jets and the Air Bleeds in the crossbar control and maintain the economical fuel/air ratios throughout this part throttle driving range. POWER SYSTEM Although the part throttle system is calibrated to give maximum economy under normal driving conditions, some means must be provided by which increased fuel is supplied when more power is desirable or sustained high speed driving is to be maintained. This is accomplished by means of a vacuum operated power piston. (See Figure 4. Note piston in "Down" position).	MODEL "BB" PART THROTTLE SYSTEM Figure 3

	MODEL "BB" POWER SYSTEM Figure 4

The power piston vacuum channel is open to the manifold beneath the throttle valves. The vacuum in this channel varies with the manifold vacuum. The power piston actuating spring is so calibrated as to force the piston down when the manifold vacuum drops below approximately 7" Hg. This situation occurs at driving speeds over approximately 75 MPH. or on rapid acceleration. When the power piston is down it unseats the spring loaded power valve permitting an additional fuel flow from the bowl through the power restrictions and into the main well channels. This additional fuel is supplied as long as the manifold vacuum remains below approximately 7 inches Hg. There is no adjustment required for the part throttle or power systems.