It sounds like you're leaking fuel through the carb vent tube. Look between the heads and you should see a rubber tube coming from between the carbs. If fuel is coming out there you have a stuck float or gummed up needles in the carb. You can try replacing the fuel filter and pouring in some Seafoam, try starting it, and letting it sit for a couple of days. If you're careful you can pull the bowls and spray some carb cleaner in there. If that does not work you will have to pull the carbs and clean them. Not hard, just a PITA.