1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 68Get this manual
1Connect timing light to no1 (top) spark plug lead2Shift engine into neutral3Holding throttle arm at idle position, crank engine with starter motor and adjust idle timing screw to align degrees BTDC timing mark of flywheel with timing pointerTighten locknut MAXIMUM TIMING 1Hold control arm against maximum advance stopCrank engine with starter motor and adjust maximum advance screw to align 25 BTDC mark on flywheel with timing pointer (due to the advance characteristics of ignition system, this cranking speed adjustment will automatically be reduced to 23 BTDC at engine speed of 5000 RPM)Tighten locknut