I had the same problem...but most because my fuel gauge wasn't working correctly and I was putting in to much gas. I was wondering why the boat was slower...turns out I was carrying 100+ gallons of fuel.
I had my mechanic look at the vent (sometimes spiders make a home in there) and it was fine. But he did say that the fuel lines/filler line and vent lines were getting old. So I had him replace them all from the tank to the carb and then the fuel filler hoses. Probably a good idea on an older boat.