I have a 1970 Honda CB 350. I have experienced the same issue. After sitting for 16 years,I totally disassembled the carbs, cleaning every passage thoroughly then blowing them out with compressed air. I re assembled and adjusted them, and the bike runs amazingly. But after an hour or so of sitting. It begins to drip fuel from the bowl vent hoses. Somehow the fuel level in the bowl keeps rising after the engine is off. I tried adjusting the mixture screws and it didnt help. I believe that after 40 years of being taken apart and put back together, the float tab that opens the needle and seat becomes bent or changed from the original setting and allows the bowls to fill too much before closing. If anyone has a definate answer or solution. Please let us know! I´m tired of of the mess and smelling gas in my garage.