OMC Stern Drives And Motors 1964-1986 Repair Manual., Page 41Get this manual
variation exists, then the low-reading cylinder should be checked by making cylinder leak testA simple leak test can made by first inserting teaspoonful of oil the spark plug opening of the low reading cylinder, and then cranking the engine distribute the oilCheck the compression again see if inserting the oil caused changeI the reading went up, then the compression loss is probably due worn ringsJf the reading remained the same, the loss may be due burnt valve VACUUM GAUGE
A vacuum gauge is relatively inexpensive piece of test equipment can be very handy in isolating mechanical troubles in an internal-combustion engineAs with compression gauge, you cannot rely soley on the actual numerical reading of the vacuum gaugeInstead, relative readings and typical actions of the needle provide clues some types of troublesNormal idle vacuum in the intake manifold ranges from 15HgOn latermodel engines, lower and less steady intake manifold vacuum readings becoming increasingly common because of the greater use of high-lift and the increase in the amount of valve overlapIn addition these factors, altitude a- vacuum gauge readingThat's rightAt high elevations, vacuum gauge will read about one inch lower for each 1,000 of elevation above sea levelAnother outside influence on the vacuum gauge is change in barometric pressureBecause of these factors, which not determined by the condition of the engine, is much more important you watch the action of the needle than actual readingAfter you have worked with vacuum gauge just times you will recognize such problem areas as sticking valves, tight valve lash adjustment, or restriction in the exhaust system