Foreign  particles  in  the  air  can  score  the  rotor, resulting in excessive air leakage. You must, therefore, keep   the   air   supply   as   clean   as   possible.   Lack   of lubrication  also  causes  scoring.  If  the  rotor  in  a hand-oiled  system  becomes  scored  because  of insufficient  lubrication,  the  equipment  could  be  at  fault, or   the   lubrication   instructions   may   not   have   been followed. To prevent problems in either a hand-oiled or pressure-lubricated  system,  check  the  piping  and  the passages to see that they are open. When scoring is not too serious, lap the rotor and body together. Use a thin coat  of  prussian  blue  to  determine  whether  the  rotor contacts  the  distributor  body. A broken spring may be the cause of an inoperative timing mechanism if a coil spring is used to maintain the rotor seal. If the spring is broken, replace it to ensure an effective  seal. An  improperly  timed  rotary  distributor  will  prevent an engine from cranking. Use the information given in the  instructions  for  the  specific  engine  to  check  the timing. PLUNGER-TYPE  DISTRIBUTOR  VALVE.—In a plunger-type distributor valve timing mechanism, the valve  requires  little  attention.  However,  it  may  stick occasionally  and  prevent  the  air  starting  system  from functioning properly. On some engine installations, the pilot air valve of the distributor may not open, while on other  installations  this  valve  may  not  close.  The  trouble may be caused by dirt and gum deposits, broken return springs,  or  lack  of  lubrication.  Deposits  and  lack  of lubrication will cause the unit valve plungers to bind and stick in the guides, while a broken valve return spring will keep the plunger from following the cam profile. Disassemble  and  thoroughly  clean  a  distributor  valve that sticks; replace any broken springs. Faulty Air Starting Valves Air starting valves admit starting air into the engine cylinder and then seal the cylinder while the engine is running. These valves may be the pressure-actuated or mechanical-lift   type. PRESSURE-ACTUATED VALVES.—In a pressure-actuated  valve,  the  most  frequent  trouble  is sticking.  The  valve  may  stick  open  for  a  number  of reasons. A gummy or resinous deposit may cause the upper and lower pistons to stick to the cylinders. (This deposit is formed by the oil and condensate that may be carried  into  the  actuating  cylinders  and  lower  cylinders. Oil is necessary in the cylinders to provide lubrication and  to  act  as  a  seal;  however,  moisture  should  be eliminated.)  You  can  prevent  this  resinous  deposit  from forming  by  draining  the  system  storage  tanks  and  water traps  as  specified  in  the  operating  instruction.  The deposit on the lower piston may be greater than that in the   actuating   cylinder   because   of   the   heat   and combustion gases that add to the formation if the valve remains open. When the upper piston is the source of trouble,  you  can  usually  relieve  the  sticking,  without removing the valve, by using light oil or diesel fuel and working  the  valve  up  and  down.  When  you  use  this method, be sure that the valve surfaces are not burned or deformed. If this method does not relieve the sticking condition, you will need to remove, disassemble, and clean the valve. Pressure-actuated starting valves sometimes fall to operate because of broken or weak valve return springs. Replacement  is  generally  the  only  solution  to  this condition;  however,  some  valves  are  constructed  with  a means  of  adjusting  spring  tension.  In  such  valves, increasing  the  spring  tension  may  eliminate  the  trouble. Occasionally the actuating pressure of a valve will not release, and the valve will stick open or be sluggish in closing. The cause is usually clogged or restricted air passages.   Combustion   gases   will   enter   the   air passageways, burning the valve surfaces. These burned surfaces  usually  must  be  reconditioned  before  they  will maintain a tight seal. Keeping the air passages open will eliminate extra maintenance work on the valve surfaces. MECHANICAL LIFT VALVES.—The  mechanical lift-type air starting valve is subject to leakage which, in general, is caused when the valve sticks open. Any air starting valve that sticks or leaks creates a condition that makes an engine hard to start. If the leakage in the air starting  valve  is  excessive,  the  loss  in  pressure  may prevent the engine from starting. Leakage in this type of valve can be caused by an overtightened   packing   nut.   The   packing   nut   is sometimes overtightened to stop minor leaks around the valve   stem   when   starting   pressure   is   applied,   but overtightening may prevent the air valve from seating. As  in  the  pressure-actuated  valve,  there  may  not  be enough return spring tension to return the valve to the valve seat after admitting the air charge. Obstructions such as particles of carbon between the valve and valve seat will hold the valve open, permitting combustion gases to pass. A valve stem bent by careless handling during installation may also prevent a valve from  closing  properly. If a valve hangs open for any of these reasons, hot combustion gases will leak past the valve and valve seat. 3-26