1988 Mercruiser 3.7 throttle issues

[gworthington73]

I just bought a 1988 wellcraft 19' classic 192 with a 3.7. When I took it out in the water, the throttle ran fine about half way open. When I try and open it up all the way it doesn't go, and boggs down. I noticed sometimes when I have it over half way open, it slowly builds its speed up and then I can open it up all the way, but not very often does this happen. What do you think is causing this?