Mercury Mariner Outboards 2.2 / 2.5 / 3.0 Service Shop Manual, Page 17Get this manual
Compression Check NOTE: Engine must be normal operating temperature (run to minutes) prior to checking compression cold engine or "dry" cylinder will result in false readings 1Remove spark plug 2Install compression gauge in spark plug hole3Crank engine thru at least compression strokes with throttle wide open, to obtain highest possible reading 4Check and record compression The compression reading should be at least 90 psi (620.5 kPa) lower compression indicates that cylinder is in some way defectivesuch as worn or sticking piston rings andor scored piston and cylinder5Compression check is important because an engine with low or uneven compression cannot be tuned successfully to give peak performanceIt is essentialtherefore, that improper compression be corrected before proceeding with an engine tune-up6Cylinder scoring: If powerhead shows any indication of overheating, such as discolored or scorched paint remove cylinder head and inspect cylinder visually for possible scoring It is possible for cylinder to be scored slightly and still have comparitively good compression