Mercury Optimax Diagnostic Guide., Page 17Get this manual
OPTIMAX DIAGNOSTIC GUIDE
Procedure 14
ENGINE COMPRESSION
Engine compression should be checked with the engine block warm, the throttle shutter wide open, all spark plugs removed and using fully charged batteryNormal compression for all cylinders should be 621 kPa758 kPa (90 psi110 psi)Cylinders should not vary more than 103 kPa (15 psi) between one anotherA variance of more than 103 kPa (15 psi) would indicate the need for power head inspectiondisassembly
CAUTION
When performing the cylinder compression test, disconnect the fuse controlling the fuel injectors to prevent fuel from being discharged
Procedure 15