Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 32Get this manual
GENERAL INFORMATION
Compression Check
1Remove spark plugs2Install compression gauge in spark plug hole3Hold throttle plate at W.O.T4Crank the engine over until the compression reading peaks on the gaugeRecord the reading5Check and record compression of cylinderA reading below 36 psi might indicate total engin wear problem
Maximum (psiY 50
Minimum (psi)