Mercury 15 hp 4 stroke won't rev over 3500

[Phoxtrot1]

I have a 2006 mercury 15 hp 4 stroke that from new has never pulled over 3500 rpm. It runs great, but just wont rev. The throttle goes to full stop, so it's getting full throttle.

My father bought it new and always complained of the lack of power, now I have it. The merc dealer told him there was no problem, but there is...It's rated at 5500 rpm.