Mercury Mariner 25HP 4-Stroke Outboard Service Manual 1997, Page 131Get this manual
Valve Clearance NOTE:Valves should be adjusted when engine is cold
POSITIONING OF TOP CYLINDER (#1) Position thepiston (top) atlop dead center (TDC) on the compression stroke1Rotate the driven gear clockwise to align the "1" mark on the driven gear with the cylinder head mark ":::,55779
Engine CQuid possibly start when turning flywheel during adjustmentTo prevent this type of
accidental engine starting and possible serious injury, always remove spark plug leads from
spark plugs1Remove flywheel cover (refer to page 4A-2)
2Remove cylinder head cover (refer to page 4A-3)
Checking Valve Clearance
IMPORTANT: Timing belt must be properly installed with timing marks on the drive gear and
the driven gear aligned with corresponding marks on powerhead1Position the driven gear for cylinderor #2 (see instructions following)aCylinder Head MarkDriven Gear Mark