1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, Page 56Get this manual
INSPECTION AND REPAI R Cylinder head 1Using rounded scraper, remove any carbon deposits from combustion chamberTake care to avoid damaging the spark plug threadsDo not use sharp instrumentAvoid scratching the aluminum2Thoroughly clean the cylinder head water jacket of salt and dirtPatch up any part from which anti-corrosion paint has come offReplace the cylinder head if its inner surface has any corroded areaCheck the cylinder head warpage with straightedge as shownThe warpage should not exceed the specified limit; if necessary, resurface the cylinder headCylinder head warpage: Less than 0.1 mm (0.004 in)