You probably have a small piece of dirt/crud in the needle valve that is keeping it open. The float, the white plastic thing in the bowl of the carburetor pushes up on the needle valve to shut off the flow of fuel when the bowl is full. If there's dirt in the seat where the needle valve goes it will allow fuel to continue to flow and cause a rich fuel to air mix as well as the conditions you describe. Do as Breno said; remove the bowl and clean it. Carefully inspect the needle and seat and wash things well with either carburetor cleaner or another solvent. Dry the parts and reassemble the float and bowl. Don't loose anything such as the pin that holds the float linkage in place.
If dirt is the culprit then you either have no fuel filter on the petcock or you're not using an in-line filter. The kit tanks are notorious for being shipped with rust and dirt inside. We always recommend cleaning the tank before the initial installation. If it shows signs of rust you can put a couple of handfuls of sand in the tank, shake it for a while, like a good martini, then rinse with hot water and dry it. BBs work well too as does pee gravel or bird shot (lead pellets), anything that will shake around inside and loosen rust scale. Good luck, let us know what you find.
Tom