Nightcruiser's advice is right on when it comes to the 'O' ring. Just because there's an 'O' ring there doesn't give you a positive seal. The carburetor must be pressed tightly against it, not just pushed on before tightening the clamp.
As for sealers, you must read the label on any product you plan to use. MOST silicone based gasket sealers are NOT compatible with gasoline. Yes, they'll seal, for a while, until the gas attacks it and it turns to goo.
The SealAll product is unbeatable. It is formulated especially to work when in contact with gasoline, even gas with ethanol. Use the RTV stuff for other things but don't expect it to last long if gasoline, or gas fumes are present.
Tom