2003 Johnson ST 55 HP WRL 2 Stroke Commercial Service Manual, PN 5005483, Page 124Get this manual
POWERHEAD SAFETY INFORMATION
SAFETY INFORMATION
Before working on any part of the outboard, first read the SAFETY section at the end of this manual
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 hole (14 mm threads)While cranking outboard with starter, note maximum pressure reading on gaugeRepeat procedure for each cylinderReturn throttle to idle position and reconnect sensor connectorReplace spark plugsVariation between cylinders should not exceed 15 psi (100 kPa)If engine shows variation greater than 15 psi (100 kPa), check for: damaged head gasket damaged pistons broken or stuck piston rings scored cylinder walls