09-10-2019 07:34 - edited 09-10-2019 07:44
If the engine is turning the same rpm with the same prop, but the speed drops 10 mph, then something has changed. Check the accuracy of the instruments, have a prop shop check the blades, make sure there isn't new bottom paint or fittings added to the hull or transom, and check if any additional weight has been added to the boat in any way. Do not overlook the possibility of excessive slippage due to turbulent or aerated water getting into the propeller blades.
Just like a doctor diagnosing a patient, you have to rule out one thing after another.
You do not have to post the same question multiple times here on this website. It is not needed plus it adds confusion at times. Please delete your other queries. Thank you.
"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.