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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.
 
                   
  Picture
         
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.
 
         
Picture    
                     
        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.
                     
    Picture
      Picture
      Picture
      Picture
           
  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.

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