TM 10-3930-671-24 FIGURE 1.11. J. AUTOMATIC ADVANCE-SPEED RESPONSIVE The  Roosa  Master  design  permits  the  use  of  a  simple, direct-acting    hydraulic    mechanism,    powered    by    fuel pressure   from   the   transfer   pump,   to   rotate   the   cam slightly and vary delivery timing. The advance mechanism  advances  or  retards  start  of  fuel  delivery  in response   to   engine   speed   changes.   In   most   injection systems,  the  actual  beginning  of  delivery  of  fuel  at  the nozzle  will  start  later  (in  engine  degrees  of  rotation)  as the speed increases. Compensating inherent injection lag improves highspeed performance of the engine. Starting delivery of fuel to the nozzle   earlier   when   the   engine   is   operating   at   higher speed  ensures  that  combustion  takes  place  when  the piston    is    in    its    most    effective    position    to    produce optimum power with minimum specific fuel consumption and minimum smoke. The  advance  pistons  located  in  a  bore  in  the  housing engage   the   cam   advance   screw   and   move   the   cam (when  fuel  pressure  moves  the  power  piston)  opposite the  direction  of  rotor  rotation  (Figures  1.12  and  1.13). Fuel   under   transfer   pump   pressure   is   fed   through   a drilled passage in the hydraulic head which registers with the bore of the head locating screw. Fuel is then directed past the spring-loaded ballcheck in the bore of the head locating  screw.  It  then  enters  the  groove  on  the  outside diameter   of   the   screw   which   registers   with   a   drilled passage in the housing leading to the power piston side of the automatic advance assembly. A  groove   around   the   power   piston   plug   and   a   drilled passage allow the fuel to enter the advance piston bore. Fuel   pressure   against   the   piston   must   overcome   the opposing spring force plus the dynamic injection loading on  the  cam  in  order  to  change  the  cam  position.    The spring-loaded ballcheck in the bore  of  the  head  locating screw prevents the normal tendency of the cam to return to the retard position during injection by trapping the fuel in  the  piston  chamber.  When  engine  speed  decreases, the hydraulic pressure is reduced and the spring returns the   cam   to   a   retarded   position   in   proportion   to   the reduction  in  speed.  The  fuel  in  the  piston  chamber  is allowed   to   bleed   off   through   a   control   orifice   located below the ballcheck valve in the head locating screw. At   low   speeds,    because    transfer    pump    pressure    is comparatively   low,   the   cam   remains   in   the   retarded position.  When  engine  speed  increases,  transfer  pump F-221