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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.
A 15 HP outboard carburetor
2 shims, part number 0325038
A 15 HP idle jet, part number 338273
It is more economical for the outboard motor company to manufacture one engine for both the under 10 HP and the above 10 HP market and just make a different carburetor, idle mechanism and air supply to make the difference between 9.9 HP and 15 HP. The engine block, the pistons and all other engine parts are identical. Only the air supply parts and the decals are different. That is why the conversion is so easy.
Bigger is not always better. Sometimes a 9.9 HP motor is better than a 15 HP motor. The 9.9 motor can go slower than the 15 HP motor. This is an advantage for manipulating a small sailboat into a dock safely. For fisherman, it is possible to do the slow trolling for relatively inactive fish like trout. Slow trolling is possible with a 9.9 HP outboard but not with a 15 HP outboard--it will stall at very low speeds.
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.