Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 119Get this manual
CHECKING VALVE CLEARANCE 1Position the driven gear for cylinder or 2See instructions preceding2Measure the valve clearance using feeler gaugeIf out of specification, refer to Adjusting Valves Adjusting Valves
1Loosen the locknut
2Insert feeler gauge (specification size) between the valve stem and valve adjusting screw3Adjust the valve adjusting screw against the feeler gauge using valve adjuster wrench until slight resistance is felt on the feeler gauge