Good move, swapping carbs! Are the carbs the AFB model with weighted air valves, or AVS (air valve secondary) with the spring-loaded valve at the top of the secondary air horn?
I'd suggest comparing part numbers, then try to find an exploded view of the carb that matches the number. It sounds like you've found the problem!
In my experience, marine engine builders have a habit of doing some really oddball things where carbs, cams, and compression ratios are concerned, particularly Mercruiser.
Jerry
