1986+ Mercury 6/8/9.9/10/15HP 2-stroke Factory Service Manual, Page 38Get this manual
GENERAL INFORMATION
Compression Check
Normal Compression (All Models) 1Remove spark plugs2Install compression gauge in spark plug hole3Hold throttle plate at W.O.T4Crank the engine over until the compression reading peaks5Check and record compression of each cylinderThe difference in compression readings between each cylinder should not vary more than 15If compression varies by more than 15 or compression for both cylinders is less than 685kPa (100psi), the powerhead should be disassembled and inspected6Compression check is important because an engine with low or uneven compression cannot be tuned successfully to give peak performanceIt is essential, therefore, that improper compression be corrected before proceeding with an engine tuneup7Cylinder scoring: If powerhead shows any indication of overheating, such as discolored or scorched paint, visually inspect cylinders for scoring or other damageRefer to Section Powerhead787 856 kPa (115 125 psi)