1980s 70hp Evinrude not running at full speed

I recently bought a 1980s 70hp Evinrude outboard. I used the existing controls and cables that where on a similar 60hp Evinrude outboard I had. With the motor off and the controls set to full throttle, the cable seems to be extended all the way out. When I took the boat out it started and idled good, it started to accelerate smoothly, but stopped accelerating and maintained a steady speed of 15 mph, when I believe it should run around 40 mph. The 16 ft boat did not even plane out. The motor sounded good and would run steady at that speed. Any suggestions as to why it is not running full speed? Thank you for your help.

Simple " no money spent " checks to do.---------Compression test.-----Next is spark test, must jump a gap of 3/8" or more on the 3 leads.--------Is timer base rotating?----If motor sat for any length of time you could have a plugged carburetor.----Do not run the motor hoping it will heal itself.---A plugged carburetor means no fuel and no oil going in 1 cylinder and can do damage.