2004 SR Johnson 2-stroke 40, 50HP Service Manual, Page 185Get this manual
POWERHEAD GENERAL
GENERAL
Cylinder Compression Testing
Start and run outboard until it achieves operating temperature, then shut OFFDisconnect ignition sensor lead from power packRemove all spark plugsAdvance throttle linkage to WOTInstall compression tester's hose attachment into spark plug h01e (14 mm threads)While cranking outboard with starter, note maximum pressure reading on gaugeRepeat procedure for each cylinderIf engine shows variation greater than 15 psi (100 kPa), check for: damaged head gasket damaged pistons broken or stuck piston rings scored cylinder walls Return throttle to idle position and reconnect sensor connectorReplace spark plugs