If the rpm limits that were posted are correct......the prop hub wouldn't have to slip but a little..... a 550 rpm difference on a two stroke would not be that noticable ( they sound like a mad bumble bee anyhow)......old motor, old prop......that could be it. But, he really needs to go to a prop shop and find out.....it does not cost just to ask them questions ya know. See what their thoughts are and go from there.