Your 9.9HP Johnson or Evinrude outboard motor is a de-tuned version of the 15HP motor for 1983 and many other model years. The 9.9HP power head and lower unit components are all rated for 15HP, but the carburetor restricts RPM's and limits the engine's power.

Convert your 9.9HP outboard to 15HP by exchanging the 9.9HP carburetor with a 15HP carburetor. The difference in these carburetors is the larger diameter of the throat opening on the 15HP. This allows for more fuel to be sucked into the engine thereby achieving higher RPM's.