a  7  to  1  compression  ratio  with  all  combustion chambers   the   same   volume,   the   compression pressure  would  be  about  120  pounds  in  all cylinders.  However,  if  one  combustion  chamber is  1/3  cubic  inch  too  small,  the  pressure  will  be about 126 pounds, and if it is 1/3 cubic inch too large, the compression pressure would be about 114 pounds. This is a variation of 12 pounds. Also note   that   a   carbon   deposit   will   raise   the compression  pressure  at  any  given  ratio  by reducing  the  combustion  chamber  volume—the greater the deposit, the higher the pressure. To  make  a  compression  test,  first,  warm  up the engine. Warming up will allow all the engine parts  to  expand  to  normal  operating  condition  and will  ensure  a  film  of  oil  on  the  cylinder  walls. Remember  that  the  oil  film  on  the  walls  of  the cylinder helps the expanded piston rings to seal the  compression  within  the  cylinder.  After  the engine is warmed to operating temperature, shut it down and remove all the spark plugs. Removing all the plugs will make the engine easier to crank while  you  obtain  compression  readings  at  each cylinder.  The  throttle  and  choke  should  be  in  a wide-open  position  when  compression  readings are  taken.  Some  compression  gauges  can  be screwed   into   the   spark   plug   hole.   Most compression  gauges,  however,  have  a  tapered rubber end plug and must be held securely in the spark  plug  opening  until  the  highest  reading  of the gauge is reached. Crank the engine with the starting motor until it  makes  at  least  four  complete  revolutions. Normal compression readings for gasoline engine cylinders are usually 100 psi or slightly higher. Compression testing is faster and safer when there are  two  mechanics  assigned  to  the  job.  Remember that  the  compression  test  must  be  completed before  the  engine  cools  off. Unless  the  compression  readings  are  inter- preted correctly, it is useless to make the tests. Any  low  readings  indicate  a  leakage  past  the valves,  piston  rings,  or  cylinder  head  gaskets. Before  taking  any  corrective  action,  make  another check   to   try   to   pinpoint   the   trouble.   Pour approximately a tablespoon of heavy oil into the cylinder  through  the  spark  plug  hole,  and  then retest the compression pressure. If the pressure increases to a more normal reading, it means the loss  of  compression  is  due  to  leakage  past  the piston   rings.   If   adding   oil   does   not   help compression  pressure,  the  chances  are  that  the leakage   is   past   the   valves.   Low   compression between two adjacent cylinders indicates a leaking or  a  blown  head  gasket.  If  the  compression pressure of a cylinder is low for the first few piston strokes  and  then  increases  to  near  normal,  a sticking  valve  is  indicated.  Near  normal compression readings on all cylinders indicate that the  engine  cylinders  and  valves  are  in  fair condition.  Indications  of  valve  troubles  by compression  tests  may  be  confirmed  by  taking vacuum  gauge  readings. Vacuum Gauge Test When an engine has an abnormal compression reading, it is likely that the cylinder head will have to  be  removed  to  repair  the  trouble.  Nevertheless, the  mechanics  should  test  the  vacuum  of  the engine with a gauge. The vacuum gauge provides a   means   of   testing   intake   manifold   vacuum, cranking   vacuum,    fuel  pump  vacuum,  and booster  pump  vacuum.  The  vacuum  gauge  does NOT  replace  other  test  equipment,  but  rather supplements  it  and  diagnoses  engine  trouble  more conclusively. Vacuum  gauge  readings  are  taken  with  the engine running and must be accurate to be of any value.   Therefore,   the   connection   between   the gauge  and  intake  manifold  must  be  leakproof. Also, before the connection is made, see that the openings to the gauge and intake manifold are free from  dirt  or  other  restrictions. When a test is made at an elevation of 1,000 feet  or  less,  an  engine  in  good  condition,  idling at a speed of about 550 rpm, should give a steady reading  of  from  17  to  22  inches  on  the  vacuum gauge. The   average   reading   will   drop approximately  1  inch  of  vacuum  per  1,000  feet at  altitudes  of  1,000  feet  and  higher  above  sea level. When  the  throttle  is  opened  and  closed suddenly, the vacuum reading should first drop to  about  2  inches  with  the  throttle  open,  and  then come  back  to  a  high  of  about  24  inches  before settling  back  to  a  steady  reading  as  the  engine idles, as shown in figure 3-10. This is normal for an  engine  in  good  operating  condition. If the gauge reading drops to about 15 inches and remains there, it would indicate compression leaks between the cylinder walls and the piston rings  or  power  loss  caused  by  incorrect  ignition timing,  A  vacuum  gauge  pointer  indicating  a steady  10,  for  example,  usually  means  that  the valve  timing  of  the  engine  is  incorrect.  Below- normal readings that change slowly between two limits,  such  as  14  and  16  inches,  could  point  to a number of troubles. Among them are improper carburetor   idling   adjustment,   maladjusted   or 3-12