1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, Page 93Get this manual
GEAR SHIM ADJUSTMENT
Preparation for adjustment Before making shim adjustment, all parts should be thoroughly cleaned and blown with compressed airShim adjustments should be made with the water pump removed
Gear backlash A" clearance(called "backlash") is allowed between the two mating gears in order to take into account possible errors in machining, center-to-center distance tolerance, deformation due to load or thermal expansion and oil film thicknessShim adjustments are required if the following parts are replacedII