Mercury Mariner 25HP 4-Stroke Outboard Service Manual 1997, Page 34Get this manual
Compression Check 1Remove spark plugs2Install compression gauge in spark plug hole3Hold throttle plate at W.O .T
Cylinder Leakage Testing
NOTE: Cylinder leakage testing", along with compression testing, can help the mechanic pinpoint the source of mechanical failure by gauging the amount of leakage in an engine cylinderRefer to the manufactures tester instructions for proper testing procedures
4Crank the engine over until the compression
reading peaks on the gaugeRecord the reading
5Check and record compression of each cylinderThe highest and lowest reading recorded should not ditier by more than 15 (see example chart below) reading below 120 psi might indicate total engine wear problem
Example of compression test differences
Maximum (psi) 180 150 Minimum (psi) 162 127.5
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