1987 Johnson/Evinrude CU Outboards 35A thru 55 Service Manual, Page 55Get this manual
Tune-Up Procedure 1Visually inspect engine for leaks, missing or loose parts, or other obvious defects2Compression checkProper compression is essential for good engine performanceAn engine with low or 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 kPa) If engine: has low but equal compression AND is hard to start AND operates poorly check the condition of cylinder head, cylinder walls, and cylinder head gasketIf engine shows variation greater than 15 psi (100 kPa), check for: sticking rings worn rings scored cylinders leaking head gasket