Low power

[Bullman]

I purchased a 24 ft pontoon over the winter w/a 2000 Merc 115 2-stroke. I can not get the motor over 19 mph. My boating friend has same size boat w/a 50 hp Merc and can go 20. Previous owner says boat would run 30-35 for him. Motor runs good w/ out any back fire or smoking, all maintenance is kept up. One other thing is my throttle cable is a little snug and I have to hold the throttle down or it will back off on its own.

Any suggestions before heading to the shop?????

[boats.net]

What is the Rpm's at full throttle?