All the engine basic health tests should be performed first, compression, ignition, carb condition, linkage and timing adjustments, etc. per the factory service manual.
A fuel pressure test at all speeds will tell if there is a pump problem or possibly something amiss in the boat's fuel system. A vacuum gauge and a piece of clear fuel line can show if there are air bubbles in the lines or a restriction. If you have older fuel lines, they may be disintegrating internally.
Be sure to obtain the factory manual or the services of an Evinrude trained technician.
"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.