1993 johnson 70hp has no power

[stever55]

I just bought a 20' pontoon with a 70hp johnson on it. They guy just had it tuned up but it still will not go very fast. Top speed is about 15mph but even more odd than that is the max rpm is 4100. The specs say it should be between 5000 and 6000 rpms. I am not very familiar with outboards so I am hoping for some suggestions. Thanks for your time

[boats.net]

It does need a smaller pitch prop to get rpm's up. 15-20 mph is probably all you would be able to get on a 20' pontoon with 70hp.