Chuck: First a question. How does the engine move the boat under load? Does it have the control that it normally has?
I have a thought regarding the racing engine. Normally if the air fuel ratio is correct an engine will idle and pull well. However if there is a vacuum leak, the idle will be unusually high. When this occurs, pull the choke slightly. If it is a lean condition it will come down with the application of the choke. I would check the carb base nuts. If one or both are slightly loose, air leaks into the manifold, the A/F ratio goes lean and the speed goes up. BUT, normally when this happens the ability of the engine to operate under load is usually impacted. Good luck and keep us all posted when you figure it out. Sail On. Al