1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, Page 97Get this manual
Measuring the backlash of reverse gear 1Repeat steps and in forward gear2Install the reverse gear holder on the propeller shaft and tighten the screwNext pull the propeller shaft and lock the propeller shaft and reserve gear3Invert the gear case and install it on the repair standInstall the dial gauge on the dial gauge stand and install it on the gear caseInstall the backlash indicator rod on the driveshaft, and make the dial gauge stem contact the "30A" on the indicator rod 4Slowly turn the driveshaft in and out and read the dial gauge when it stops in each directionDetermine the shim size according to the following chartThickness of shims to be added or removed Measurement is less than specification: (0.98measurement)thickness of shims to be added (Measurement x2= thickness of shims to be removed