FUEL SYSTEM - N.15 Fig. N.23. Slackening Union Nut at Pump Inlet. the excess fuel return pipe, to the fuel tank, from the   filter   head.      Operate   the   fuel   feed   pump priming   lever   until   fuel,   free   from   air   bubbles issues   from   the   non   return   valve.      Refit   fuel return pipe. 2. Slacken  the  vent  screw  on  the  hydraulic  head locking  screw  on  the  side  of  the  fuel  injection pump body.  See Fig. N.20. 3. Slacken  the  air  vent  screw  on  the  side  of  the governor control cover.  See Fig. N.21. 4. Operate the priming lever of the fuel lift pump — see  Fig.  N.22  (note  —  if  the  cam  on  the  engine camshaft     driving     the     fuel     lift     pump     is     on maximum  lift,  it  will  not  be  possible  to  operate the    hand    primer    and    the    engine    should    be turned  one  complete  revolution)  and  when  fuel, free   from   air   bubbles   issues   from   each   vent point,   tighten   the   connections   in   the   following order. (a) Filter cover vent screw. (b) Head locking screw vent valve. (c) Governor cover vent valve. 5. Slacken the pipe union nut (see Fig. N.23) at the pump    inlet,    operate    the    priming    device    and retighten  when  oil,  free  from  air  bubbles  issues from around the threads. 6. Slacken  the  unions  at  the  atomiser  ends  of  two of the high pressure pipes. 7. Set the accelerator at the fully open position and ensure   that   the   "stop"   control   is   in   the   "run" position. 8. Turn    the    engine    until    fuel    oil,    free    from    air bubbles, issues from both fuel pipes. 9. Tighten  the  unions  on  both  fuel  pipes,  and  the engine  is  ready  for  starting.    If  fuel  has  been drained  from  the  thermostat  feed  pipe,  the  pipe must  be  disconnected  at  the  thermostat  and  all air  bled  from  the  pipe  before  the  thermostat  is operated. Priming Procedure after changing a Filter Element 1. With  the  vent  plug  on  the  filter  cover  removed, and  the  union  at  the  filter  end  of  the  return  pipe (filter to tank) slackened, operate the feed pump priming   lever   until   oil,   free   from   air   bubbles, issues from the filter cover vent. 2. Replace the  vent  plug,  and  continue  to  operate the  priming  lever  until  oil,  free  from  air  bubbles, issues   from   around   the   threads   of   the   return pipe union. 3. Tighten the return pipe union. 4. Slacken  the  union  at  the  filter  end  of  the  filter  to injection    pump    feed    pipe,    and    operate    the priming   lever   until   oil,   free   from   air   bubbles, issues from around the union threads. 5. Tighten the feed pipe union.  The pump and filter are  now  filled  and  primed  and  ready  for  further service. 6. Later  type  fuel  filters  may  not  have  an  air  bleed plug.  In this case remove the excess fuel return pipe,    to    the    fuel    tank,    from    the    filter    head. Operate  the  feed  pump  priming  lever  until  fuel free  from  air  bubbles  issues  from  the  non-return valve.  Refit fuel return pipe.  Continue from item 4.