late 80s 30 hp problems. help!!

hey guys I just buy a late 80's 30 horsepower Evinrude it seems to be a good motor. I checked compression in cylinder and both read right at 120 and the lower unit is good and clean no water the previous owner said he and carb rebuild seals put in lower unit and just general maintenance gone through about 2 or 3 years ago and the motor has been sitting since then until now I took it and ran it in a barrel and ran good and I took it to the water on my 16 foot aluminum jon boat and I am very disappointed in the performance I had. I had a 20 horse that push me 18 and 19 with two people and the 30 was pushin me and another guy with 2 batteries 16 miles an hour I had assumed it would push you at least 20 if not a little better with just me in it and ran 18 these numbers seem low is there anything that I need to do that can be done to get some more performance out does anything need to be cleaned I'm hoping it's just because Iits been sitting for awhile and it needs is a tune up thanks for any help

Did the previous owner ever run it on the water after rebuilding it? If not the carb might not be adjusted after having been pulled apart - otherwise, is it installed to the proper angles & clearances under the boat?