TM 1-1500-204-23-1 NOTE When adjusting clearance and staking  valve  do  not  correct  blowby and raise compression above minimum  value  as  specified.    In  the applicable maintenance manual, replace    cylinder    as    described    in applicable maintenance manual. (d) Compression test. A cylinder compression test locates defective cylinders. Replacement of these defective cylinders will prevent an unsatisfactory  condition  from  progressing  to  the  point  of complete   cylinder   failure   and   eventual   engine   failure. Cylinders    with    compression    below    standard    shall    be replaced, thus preventing the use of engines with a large variation of compression between individual cylinders. CAUTION Ensure that Ignition switch is placed in OFF position after engine warmup has   been   accomplished.      Damage may otherwise result 1 When  engine  Is  cold,  operate  it  a short  time  to  ensure  that  cylinder  walls  and  piston  rings are properly lubricated. 2 Remove one spark plug from each cylinder and insert compression tester. 3 Either    disconnect    starter    booster coil or ground all disconnected spark plug lead. 4 Place throttle in FULL open position, keep fuel tank selector valve closed. 5 Use  battery  cart  or  auxiliary  power unit to provide power for energizing engine starter. 6 Using  engine  starter,  crank  engine at least four revolutions. NOTE The    tester    contains    a    restrictive orifice to eliminate acceleration errors, and two compression impulses  are  required  to  obtain  an accurate compression reading. 7 Analyze compression reading obtained. 8 Determine average cylinder compression values by totaling all cylinder compressions and dividing total obtained by number of cylinders.  If any one  cylinder  is  20  percent  below  the  average,  or  If  any one cylinder is below the minimum compression value for that engine, that cylinder must be replaced. NOTE A variance of any one cylinder of 20 percent  below    the    average    of    all cylinder    compression    readings    of engine being checked warrants additional investigations. Two additional   checks   should   be   made prior  to  any  detailed  work  to  verify information obtained In first compression check. 9 The following conditions may possibly affect engine compression. Incorrect valve tappet clearances Worn, scuffed, or damaged pistons Broken piston nng Excessive  wear  of  piston  rings  and  cylinder walls Burned or warped valves Carbon   particle   between   face   and   seat   of valve Incorrect valve timing NOTE When  a  compression  test  on  one  or more cylinders of any row of cylinders is needed, the compression of  all  cylinders  on  the  affected  row will  be  tested,  compression  reading totaled,   and   minimum   compression established  in  the  same  manner  as when   checking   compression   on   all cylinders 7-11