HELP HELP!!! 192 American ran great for three hours then started to bog out in gear

Hi this is my first ever post here, I read the comments on here all the time and are so helpful. So hopefully you can help me now? I have a 1986 Wellcraft 192 American just changed the engine from a 170hp to a 165hp finally got it running. Just rebuilt the carb adjusted the time and was off. Took out the kids for some tubing. Ran great for about 3 hours then it would lose power. I would back off the throttle so the engine wouldnt die and then I would engage drive again with the same result. I can run with little throttle applied but it will sputter out almost at half the throttle. I was looking down in the carb while running and noticed that the throttle valve at the bottom isn't opening. It does however open all the way with the motor shut off. What would cause this I am completly lost. All my Daughter wants for her Birthday is to take her friends out on the boat. Someone please help me...