Center Console Boats And More

What can I do to get proper WOT on 35 HP Mercury?

Boat is a 24' pontoon,prop is 12x9,current rpm is 2900.Prop is new,tach is new. Motor runs great,starts great,idles great,just can't get up to 5000-5500. Motor will rev to rated WOT in cold start,but will not in gear.This is an older boat and motor-1986.