1995 Mariner Mercury Outboards Service Manual 50HP 4-Stroke, Page 20Get this manual
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 reading 5Check and record compression of each cylinderVariation of more than 15 psi(103.5 kPa) between cylinders indicates that lower compression cylinder is in some way defective, such as worn or sticking piston rings andor scored piston and cylinder andor worn or burned valvesNOTE: Normal Compression should be approximately
ANALYSIS:
Due to standard engine tolerances and engine wear, no cylinder will maintain of leakageIt is important only that cylinders have somewhat consistent reading between themDifferences of 15 to 30 indicate excessive leakageLarger engines tend to have larger percentage of cylinder leakage than smaller enginesIf excessive leakage is present, first check that the piston is at top dead center of it's compression strokeLeakage will naturally occur if the exhaust or intake valve is openTo determine the cause of high percentage leaks, you must locate where the air is escaping fromListen for air escaping thru the carburetor intake, adjacent spark plug holes, exhaust pipe, crankcase fill plugUse the following table to aid in locating the source of cylinder leakage: Air Escaping From: Indicates Possible Defective: