sounds like the previous owner was onto something or they already knew what the problem is.best advice i can give you on this is to rebuild the carb if possible
sounds like it is filthy on the inside.beeing a boater myself smart money is not to take any chances out there
make sure it's running optimum it's worth not getting
stuck and waisting a day off and more money. and most of all not endangering yourself or guests with.

good luck
hunter