Yamaha Outboard F15A F9.9C, FT9.9D F15 Service Manual, Page 240Get this manual
POWR CAMSHAFT AND VALVES
2 The valve seat is in the middle of the valve face, but is too narrow
Valve seat cutter set Use 45 cutter
Desired result To achieve uniform valve seat width
3 The valve seat is too narrow and is near the valve margin