Mercury Mariner 4, 5, and 6HP 4-Stroke Factory Service Manual, Page 75Get this manual
TIMING AND SYNCHRONIZATION
Check Timing
1Connect timing light (91-99379) to engine2Start engine and allow it to warm up to normal operating temperature3Throttle engine back to idle and shift into "Forward" gear4With engine idling in "Forward" gear, use timing light and check flywheel timing mark alignmentIdle timing should be 25 B.T.D.C5With engine in "Forward" gear, fully advance throttle to W.O.TpositionUse timing light and check that timing mark of flywheel maintains alignment with crankcase covercylinder block 25 B.T.D.Ctiming mark