I suggest you begin by verifying that the carburetor is flooding, as explained previously. If it is, there are two likely immediate causes, as Joe said: either the needle and seat is not closing properly, or the float has sprung a leak. In my experience the needle and seat not closing is the more likely problem, and it is usually due to dirt under the needle. The dirt comes in from the fuel tank through the fuel line. You need to clean out the tank and line, and verify a steady flow of clean fuel out of the fuel line. Once that is done, you will need to clean out the carburetor, which is a bit trickier and more delicate so I suggest you do these preliminary steps first. (It would also be irritating to clean out the carburetor, put it back on the engine, turn on the fuel, and refill the carburetor with crud from the dirty fuel tank.)
