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Acceleration Pump System
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TM-5-4210-230-14P-1 Aerial Ladder Fire Fighting Truck Manual
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Deceleration  Throttle  Modulator  System  -  1973  California and 1974 thru 1980 Federal Vehicles Over 6000 Lbs.  GVW
TRUCK GROUP SERVICE MANUAL TM 5-4210-230-14&P-1 GENERAL Figure 15  Acceleration Pump Assembly (shown with plastic pump inlet valve used on later model carburetors). 1.  Diaphragm Cover 3.  Plastic Valve 2.  Diaphragm Return 4.  Diaphragm Assembly Spring diaphragm.    Hydraulic  pressure  forces  fuel  in  the  pump  cavity and  channels  past  the  needle  check  valve  and  out  the  pump discharge nozzles into the air stream. Upon return of the throttle lever toward idle position, the diaphragm   spring   returns   the   diaphragm   and   the   resultant pressure differential causes the discharge valve to close and the inlet  ball  or  plastic  check  valve  to  open,  allowing  the  pump cavity to refill with fuel from the bowl. Choke System The  function  of  the  choke  system  is  to  provide  a  rich air/fuel mixture necessary for cold starting and engine warm-up (Fig.  16). The choke plate, which is manually operated on governor type  carburetors,  may  be  closed  during  the  engine  cranking period  and  partially  opened  during  the  warm-up  period.    This induces  a  low  pressure  on  the  fuel  metering  system,  causing both    the    main    and    idle    systems    to    discharge    more    fuel producing mixtures rich enough to insure smooth power as the engine warms up. During the full choke period, the high point of the fast idle cam  is  in  contact  with  the  fast  idle  adjusting  screw  which  sets the throttle plates at a predetermined angle causing engineRPM to be high upon starting.  As the choke is manually opened, Figure 16  Manual Choke System 1.  Choke Plate 6.  Choke Cable 2.  Fast Idle Adjusting Screw   7.  Fast Idle Cam 3.  Fast Idle Pin 8.  Choke Rod Lever 4.  Throttle Shaft 9.  Choke Rod 5.  Choke Lever the  cam  moves  rotationally  away  from  the  fast  idle  adjusting screw   allowing   the   engine   to   resume   its   normal   idle   speed setting. Throttle   Modulator-1970-1971   Vehicles   Under   6000   Lbs. GVW To    provide    acceptable    exhaust    emission    levels,    the carburetors   of   certain   vehicles   require   the   use   of   a   throttle modulator. The throttle modulator is vacuum operated and functions above   a   predetermined   road   speed   to   aid   in   the   control   of exhaust emission during engine deceleration. Fig.   17   shows   throttle   modulator   diagram.      A   speed sensor  (spinner  valve)  is  driven  by  the  speedometer  cable.    A vacuum source and bleed line are piped into the speed sensor. The  sensor  is  designed  to  open  the  bleed  line  to  the  vacuum source  when  vehicle  road  speed  is  below  the  predetermined cutoff speed (approximately 24 MPH).  As long as the bleed line is open, no vacuum is available to extend the modulator. When the vehicle passes the road speed cutoff point, the bleed  line  is  closed,  making  vacuum  available  to  actuate  the modulator. CGES-125-T  Page 10 PRINTED IN UNITED STATES OF AMERICA

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