Hi I have a 1999 75hp 2 stroke Mercury outboard on a 16ft boat which I have had since the motor was new. Motor has about 300 hours on now and was recently serviced. Motor has nto had any issues until recently and normally takes the boat up to about 35kts.
When I had it serviced recently I told the mechanic that it started well, and ran well with good power (topping out over 4000 rpm). However, after about 15 minutes at planning speed (3000rpm +) it would lose power. Motor would keep running, but revs dropped back to about 2000rpm. No alarms are going off
He serviced it, replaced the plugs etc and took it for a test run. He could find no problems. However, when I took the boat out again, the same prblems happened. I thought it may have been stale fuel so I disconnected the underfloor tank and connected the motor directly to the sapre tank that has new fuel in it. It made no difference. If I idle around for about 15 minutes, or stop the engine for about 15 minutes, I get full power back for a very short period.
I would appreciate any help/advice that can help diagnose what is causing this. Thanks in advance. Regards