Things You'll Need
Instructions
Remover the carburetor from the 9.9 HP motor. The primary difference between a 9.9 HP motor and a 15 HP motor is the carburetor. Changing the carburetor does, however, entail a few other changes.
Take off the top plate and add one shim--part number 0325038--for each cylinder. The shims go behind the leaf valve and under the top plate. You can buy the shims at the outboard motor dealership.
Replace the top plate. Install the 15 HP carburetor. Looking from the rear, with the throttle plate in a horizontal position, the internal throat diameter should be 0.875 inches. If this measurement is off by more the 0.2 inches, the carburetor is on crooked. If the internal diameter is 0.675, you may have installed a 9.9 HP carburetor instead of a 15 HP carburetor.
Exchange the idle jet for a 15 HP motor. The part number for a 9.9 idle jet is 388936. The part number for a 15 HP idle jet is 338273.