FUEL SYSTEM - N.13 pressure   before   removing   from   the   Atomiser   Testing Pump. When the atomiser has been cleaned, reassembled, the  breaking  pressure  set  to  the  recommended  figure and   the   spray   formation   found   to   be   satisfactory,   the following two checks may be carried out on the Atomiser Testing Pump before returning the atomisers to service. (1)   Back Leakage Pump   up   sufficient   pressure   to   raise   the   nozzle needle from its seat, pump again slowly to just below this pressure   (approximately   160-165   atmospheres),   then upon releasing the hand lever and allowing the pressure to  fall  naturally,  record  the  time  (with  the  aid  of  a  stop- watch) taken for the pressure shown on the gauge to fall from  150  to  100  atmospheres.    For  a  nozzle  in  good condition this time should not be less than 6 seconds. When carrying out this test observe  that  no  leakage occurs at the lapped pressure faces of the nozzle holder and  nozzle  body.    Leakage  may  be  external,  when  it  is visible at the nozzle cap nut screw thread, or internal, in which    case    it    cannot    be    readily    distinguished    from excessive leakage past the lapped portion of the needle. If  leakage  past  the  lapped  portion  is  suspected,  do  not overtighten the cap nut in an effort to cure such leakage, but   remove   the   nozzle   and   re-examine   the   pressure faces  for  signs  of  dirt  or  surface  imperfections.    Clean thoroughly,     and     if     all     appears     in     order,     replace components and re-test. If  the  pressure  drop  time  is  still  low,  this  indicates excessive  leakage  past  the  lapped  portion  of  the  nozzle needle. (2)   Seat Tightness Wipe   nozzle   tip   dry,   pump   up   the   p ressure   to approximately 10 atmospheres below the nozzle opening pressure, the nozzle tip must remain substantially   dry   and   there   must   be   no   tendency   for blobs  of  fuel  to  collect  or  drip.    A  slight  dampness  may be ignored. NOTE:      If,   after   carrying   out   the   aforementioned atomiser  maintenance,  satisfactory  results  are  not obtained, the nozzle assemblies concerned may still be  fit  for  further  service  after  reconditioning.    This, however,   requires   specialized   equipment   and   the complete nozzle assemblies should be forwarded to the nearest depot or workshop capable of undertaking   such   work,   and   a   replacement   unit obtained. A   perfect   atomiser,   when   tested   by   pumping   fuel through  it  in  the  open  air  gives  a  short  "pinging"  sound as  the  fuel  emerges  from  the  holes.    After  the  atomiser has been in service for some time, the "pinging" changes to a crackling sound.  It is not until the atomiser sounds "dead"  that  its  condition  is  likely  to  affect  the  running  of the engine. When  replacing  the  atomiser  in  the  cylinder   head follow  carefully  the  instructions  for  fitting  given  on  Page N.8. Atomiser Identification Later  atomisers  have  the  identification  letters  "CU" rolled on the atomiser body. With  earlier  atomisers,  the  identification  letters  were stamped  on  the  tab  washer  fitted  under  the  spring  cap locknut. Fuel Pipes No two of the pressure pipes, from the fuel pump to the   atomisers   are   alike.      Keep   this   in   mind   when replacing. Examine  the  olives  at  each  end  of  the  pipe.    If  the union nuts have at any time been overtightened there is a  risk  that  the  olives  will  have  cracked  or  been  unduly compressed.    If  so,  leakage  will  result  and  a  new  pipe should be fitted. High   pressure   fuel   pipes   are   now   supplied   with formed   ends   in   place   of   olives.      Earlier   pipes   were supplied    with    olives    fitted    as    shown    in    Fig.    N.18. Originally,  the  olives  were  fitted  in  the  reverse  position but both positions are still satisfactory if undamaged. Fig. N.18. In   this   connection   bear   in   mind   that   the   working pressure   which   these   joints   must   sustain   is   several thousands pounds per square inch.  Only a perfect joint is satisfactory. Offer   up   the   pipe   to   the   fuel   pump   and   atomiser unions  to  check  that  the  pipe  fits  square  at  both  ends. Do  not  fit  one  and  then  bend  the  pipe  to  square  it  with the other union. When fitting the pipe tighten the unions alternately a little at a time, first one end and then the other. The  correct  tightening  torque  for  the  high  pressure fuel pipe nuts is 15 Ibf ft (2,1 kgf m) - 20 Nm. If the olives are in good condition, and the