I just bought a 06 Fountain 29ft Fever with a Mercruiser 496HO in it. The outdrive is a Bravo 1 with a 1.50 gear ratio and a 28 pitch Bravo 1 prop. Fountain says the boat should do around 70mph but I can only get it up to 61mph. At top speed the RPM is around 4900 to 5000. Any suggestions??

Are you sure you don't have a 1.65 drive ratio? With a 1.5 drive, 28P @ 5000 you should be going mid to upper 70's.

I'm not 100% sure but I think it's a 1.5. How would I be able to find out?

You should be able to find out by the serial number.

A 1.65 drive, 28P @5000 rpm with 12% slip comes out to 70 MPH.

If you do have a 1.5 drive, your slip is way too high.

Forgive me I'm new to this but what do I need to do if my slip is to high?

Slip is the percentage of forward travel you are losing due to many variables.
Your propeller has a stated pitch size. In this case, 28. Therefore the propeller should move forward 28 inches in one revolution. If you have 10% slip, you are losing 2 inches of forward travel with each revolution.
Your hull typically has higher slip numbers than most will expect.

B1 props are aprox. a inch low in pitch. I figure you slip to be in the 21-29% range. This would be abnormally high. You should be in the 12-14% range.

Brett