Ok so I have 08 df250ss. It starts up first crank, runs great throughout the rpm range until I hit 5000rpm. Once i get to 5000rpm it will not go any higher. Sometimes it will go up to 5100-5200rpm but not surging or anything. 5000rpms is basically 3/4 throttle and when I push the throttle down further, there is no change in rpms and it continues to run flawlessly. The only thing that changes is the tone of the engine, kind of has a more throaty sound like the butterfly is opening up more so its getting more air, but not any more fuel. I know my props and it's definitely not a prop issue. Motor used to run at 6000-6200rpm until one day it just wouldn't go over ~5000rpm. I have run it like this for probably 40-50 hours. I have changed water separator, vst filter, and spark plugs but nothing has changed.
