applied, the secondary throttle plates are opened when the primary throttle shaft linkage overcomes the tension of the wound spring on one end and allows the other end of the spring to force the linkage connected by a rod to the cam on the secondary throttle shaft to move.

NOTE

For detailed distributor information, see Holley Manuals B-235 and B-245.

As accelerator pedal pressure is de creased, the tension on the spring end is relaxed and the spring pressure is exerted on linkage connected to the secondary throttle shaft causing the secondary throttle plates to be mechanically closed.

Vacuum Operated Automatic Transmission Passages

C. VACUUM OPERATED AUTOMATIC TRANSMISSION - The pressure differential (vac uum) that is necessary to operate the automatic transmission on some vehicles is supplied through the carburetor. Vacuum taken acts on the automa tic transmission which controls the shifting of the gears.

Spark Advance

B. SPARK ADVANCE SYSTEM - The vacuum port is located just above the full closed position of the throttle plates. It is subjected to intake manifold vacuum as the plates are opened. The port is connected to the main body by a short vertical passage and in turn leads to a passage in the main metering body. This passage leads to an outlet on the side of the main metering body which connects to tubing leading to the distributor diaphragm. The pressure type distributor has an additional port in the venturi and uses both manifold and venturi vacuum to control the distributor for the entire range of engine opera tion.

Over-run Control Valve

D. OVER-RUN CONTROL VALVE - Some applications of truck carburetors are equipped with an over-run control valve which assists in eliminating popping back on deceleration. This valve is located in the secondary metering body.