On my CM400C I have two carbs with a long tube at the bottom of each. In the spring, when I get the bike started for the first time, I have to turn a small screw at the bottom of the carb, (near the tube) and the gasoline drains out through the tubes. After a few moments, I turn the screws tight again and the gas stops. Could these tubes of yours be the same? If so, try to turn the small screws clockwise to close the valve.