1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 124Get this manual
CARBURETOR
Carburetor Adjustments
Idle Speed
NOTE: Before adjusting the idle speed make sure the throttle link rod has been properly adjusted1Measure the idle speed using the procedure below, adjust if necessary: Start engine and allow it to warm upAttach tachometer to the spark plug lead of cylinder #1Measure idle rpm with outboard in neutralTurn throttle stop screw in direction or until the specified idle speed is obtained