2003 ST 4 Stroke 9.9/15HP Johnson outboards Service Manual, Page 43Get this manual
PERIODIC MAINTENANCE CYLINDER COMPRESSION
CYLINDER COMPRESSION
Compression pressure specification (with decompression system) Standard: Approx400 kPa (4.0 kgcm2, 57 psi) with recoil starting The above specification is guideline only, not absolute service limitsLow compression pressure can indicate one or more of the followingExcessively worn cylinder wall Worn-down piston or piston rings Piston rings stuck in grooves Poor seating of valves Ruptured or otherwise defective cylinder head gasket