1985 OMC 65, 100 and 155 HP Models Commercial Service Manual, PN 507450-D, Page 61Get this manual
Tune-Up Procedure 1Visually inspect motor 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 to normal operating temperatureStop engine
To prevent accidentally starting motor, disconnect statorcharge coil assembly to power pack connectorsSee Section Remove and inspect all spark plugsCheck their condition nstall thread-type compression tester in spark plug holeWith throttle in wide open position, crank engine with starter through at least four compression strokesVariation should not exceed 100 kPa (15 psi) between cylinders
If motor: 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 motor shows greater than 100 kPa (15 psi) difference, check forsticking rings worn rings scored cylinders leaking head gasket