No power/throttle

I have with my 40 hp Merc. it wont rev over 2600 rpm in the water while in forward, but if I rev it in neut or rev it revs to 5200+ rpm. It will rev to 5000+ rpm while in forward when its hooked up the muffs on the hose but not on the water. It ran a little rough one weekend so I ran some seafoam thru it while hooked up to the hose and took it out the next weekend find I had this current problem. Any ideas. The plugs seem fine and the compression seems fine.