I have a kohler 25 GDLT. I'm turning just over 4000 rpm in water but only just under 3000 in water. The prop is good and I have checked the timing. Motor runs smooth and fires on both cylinders. What is going on with this thing? Any help would be appreciated.

Lean from dirty carb, in need of valve job, dirty ail filter. Aside from the easy stuff u might get a leakdown test done. Also ive had to replace several flat cams on these motors. Pull valve covers and check rockers for full lift

Thanks for the suggestions.

If you eliminate those issue, maybe it's a warped upper bearing on the shaft. I had similar issues with my 25 GD LT and couldn't ever get it right, but there was nothing wrong with the engine. I sold it to a mechanic who must have known what was what, next time I bump into him I'll ask him how it's working.

See my thread too for similar discussions
viewtopic.php?f=3&t=64343

Prop?