Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 194Get this manual
Pinion Gear and Forward Gear PINION GEAR 1Inspect pinion gear teeth for breakage, rust, chipping or excessive wear2If pinion gear teeth are damaged, inspect FORWARD and REVERSE gear for damage3Replace gears as requireda
FORWARD GEAR 1Inspect forward gear teeth for breakage, rust, chipping and excessive wear2Inspect forward gear clutch jaws for wearRounded jaws indicate the following: aImproper shift cable adjustment or linkagebEngine idle speed too highcShifting too slowlya