1998 Johnson Evinrude "EC" 25, 35 HP 3-Cylinder Outboards Service Manual, Page 32Get this manual
Tune-Up Procedure 1Visually inspect engine for leaks, missing or loose parts, or the obvious defects2Compression checkProper compression is essential for good engine performanceAn engine with uneven compression cannot be properly tuned Operate the engine until it reaches normal operating temperatureStop engine Remove and inspect all spark plugsCheck their condition Install thread-type compression tester in spark plug hole With throttle in wide-open position, crank engine with starter through at least four compression strokes Variation between cylinders should not exceed 15 PSI (100 kPalIF the engine has equal compression, is hard to start, AND operates poorly, check for: scored cylinder walls damaged pistons stuck piston rings worn piston rings
IF the engine shows variation greater than 15 PSI (100 kPal, check for: damaged head gasket damaged pistons broken or stuck piston rings scored cylinder walls