If the motor was used in saltwater, the operator's guide says to lubricate the motor annually. If in fresh water that is normally not done until the 3 year/300 hour service - whichever comes first.
The top rpm being too low also may have contributed to carboning up of the exhaust tuning valve. What size boat, top speed, and what size prop is on the motor?
Email Evinrude with the engine serial number, dealer name, and what you wrote here. They will look into your situation.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.