Mercury Mariner 25HP 4-Stroke Outboard Service Manual 1997, Page 92Get this manual
Idle Speed Adjustment NOTE: The engine should be complelely warmed up for the adjustmentCorrect adjustments can not be obtained in cold condition
1With the outboard in the water, start engine and allow to warm upShift the outboard to neutral2With the outboard in neutral, turn the idle adjustment screw to obtain the specified idle RPM