DF40 2008 No longer reaching 6000 RPM only 5200

[simogoes]

I have a DF40 which has been running great. Starts all the time, has a healthy water cooling system etc. While out boating a few month ago i noticed that i couldnt get it past 5000RPM and 17kts when normally it would do 6400 RPM and 24-25kts. This issue went away next time i boated but yesterday it came back. I checked the fuel and squeezing the build didn't make any difference. The engine isnt spluttering or surging and runs smoothly, it just seems to not have the top end speed/revs. Any suggestions?

The only other thing i notice with it, when flushing in idle, if you move the throttle lever to rev it in idle, it can't get above 3000rpm without spluttering and coughing....not sure if its related but in gear it doesn't do that.

[Moonlighter]

The engine has a protection system built in that prevents it being revved past 3000rpm in neutral. So that is working exactly as it should.

Change the spark plugs and I suspect your problem will go away.