Blown stern drive boats or big geared day cruisers boost ref the power valves in the carbs. The blower draws hard enough to suck the power valves shut. (sometimes) Especially engines that get in boost during cruise. power valves are supposed to be referenced to manifold pressure and not top of the blower pressure.
Let me see if I can find a link to the proper proceedure. You just have to port manifold pressure to the power valve wells.