I feel your carburettor has a faulty float. It allows the gas to keep flowing till the engine is full or flooded. The float is made to cut the flow of gas off when the engine shuts down.

Yup float is stuck down slightly... probably a piece of old gas stuck in the gas intake or at the side of the floats pin restricting it's free movement. Using a gas stabilizer like seafoam stops this issues I have seen plastic floats that have a leak become saturated with gas and have the same issue.