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How to Turn a 9.9 HP Outboard Into a 15 HP Motor

by Carlos Mano ; Updated April 12, 2017

Small outboard motors are very useful.

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Small outboard motors are used to take fishermen to their fishing sites and to move small sailboats into and away from docking areas. Because state and federal laws distinguish between motors that have less than 10 horsepower and more than 10 horsepower, most companies make an outboard motor in both size ranges. The most common sizes are 9.9 horsepower and 15 horsepower. Owners found out long ago that it is fairly easy to convert a 9.9-horsepower motor to a 15-horsepower motor.

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.

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