TM 10-3930-671-24 due  to  the  cam  rollers  passing  the  highest  point  on  the cam lobe. Following this, the rotor discharge port closes completely and a residual injection line pressure is maintained. Note that  the  delivery  valve  is  only  required  to  seal  while  the discharge   port   is   opened.   Once   the   port   is   closed, residual line pressures are maintained by the seal of the close fitting head and rotor. G. RETURN OIL CIRCUIT Fuel  under  transfer  pump  pressure  is  discharged  into  a vent  passage  in  the  hydraulic  head  (Figure  1.10).    Flow through    the    passage    is    restricted    by    a    vent    wire assembly   to   prevent   excessive   return   oil   and   undue pressure  loss.  The  amount  of  return  oil  is  controlled  by the  size  of  wire  used  in  the  vent  wire  assembly,  i.e.  the smaller the wire the greater the flow and vice versa.  The vent wire assembly is available in several sizes  in  order to    meet    the    return    oil    quantities    called    for    on    the specification.  Note  that  this  assembly  is  accessible  by removing  only  the  governor  cover.  The  vent  passage  is located  behind  the  metering  valve  bore   and   connects with  a  short  vertical  passage  containing  the  vent  wire assembly and leads to the governor compartment. Should a small quantity of air enter the transfer pump, it immediately  passes  to  the  vent  passage  as  shown.    Air and a small quantity of fuel then flow from the housing to the fuel tank and via the return line. Housing    pressure    is    maintained    by    a    spring-loaded ballcheck return fitting in the governor cover of the pump. H. MECHANICAL ALL SPEED GOVERNOR The   governor   serves   the   purpose   of   maintaining   the desired  engine  speed  within  the  operating  range  under various load settings. In the mechanical governor (Figure 1.11), the movement of  the  weights  acting  against  the  governor  thrust  sleeve rotates the metering valve by means of the governor arm and linkage hook. This rotation varies the registry of the metering valve opening to the passage from the transfer pump,   thereby   controlling   the   quantity   of   fuel   to   the plungers.  The  governor  derives  its  energy  from  weights pivoting   in   the   weight   retainer.      Centrifugal   force   tips them outward, moving the governor thrust sleeve against the governor arm, which pivots on the knife edge of the pivot    shaft    and,    through    a    simple,    positive    linkage, rotates   the   metering   valve.   The   force   of   the   weights against  the  governor  arm  is  balanced  by  the  governor spring    force,    which    is    controlled    by    the    manually positioned    throttle    lever    and    vehicle    linkage    for    the desired engine speed. In the event of a speed increase due to a load reduction, the resultant  increase  in  centrifugal  force  of  the  weights rotates the metering valve clockwise to reduce fuel. This limits the speed increase (within the operating range) to a value determined by governor spring rate and setting of the throttle. When  the  load  on  the  engine  is  increased,  the  speed tends   to   reduce.   The   lower   speed   reduces   the   force generated  by  the  weights  permitting  the  spring  force  to rotate    the    metering    valve    in    the     counterclockwise direction to increase fuel. The speed of the engine at any point  within  the  operating  range  is  dependent  upon  the combination   of   load   on   the   engine   and   the   governor spring   rate   and   setting   as   established   by   the   throttle position. A light idle spring is provided for more sensitive regulation  when  weight  energy  is  low  in  the  low  end  of speed   range.      The   limits   of   throttle   travel   are   set   by adjusting    screws    for    proper    low    idle    and    high    idle positions. A  light  tension  spring  on  the  linkage  assembly  takes  up any slack in the linkage joints and also allows the shutoff mechanism to close the metering valve without having to overcome the governor springing force.  Only a very light force   is   required   to   rotate   the   metering   valve   to   the closed position. FIGURE 1.10. F-220