Honda Outboards BF40A/BF50A Service Manual, Page 114Get this manual
HONDABF35A45A
VALVE SEAT RECONDITIONING
1Thoroughly clean the combustion chambers and valve seats to remove carbon depositsApply light coat of Prussian Blue compound or erasable felt-tipped marker ink to the valve faces2) Insert the valves, and then lift them and snap them closed against their seats several timesBe sure the valve does not rotate on the seatThe transfered marking compound will show any area of the seat that is not concentricNOTE
Follow Valve Seat Cutter Manufacturer's Instructions
3) Using 45 cutter, remove enough material to produce smooth and concentric seatTurn the cutter clockwise, never counterclockwiseContinue to turn the cutter as you lift it from the valve seat