Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 132Get this manual
Condition: Valve seat is to narrow and it is near valve margin Oil Separator
1Check reed valves for warpageReplace if out of specificationg k
Valve Seat Cutter Set Use Lightly 15 Cutter, First 45 Cutter
Desired Results To center the seat and to achieve its Width
Condition: Valve seat is to narrow and is located near the bottom edge of the valve face