I will take a guess that you have a stuck needle valve. The needle valve starts and stops fuel coming into the float bowl in the carburetor. It works off a float in the carb bowl. If it sticks open, it allows fuel to flood the carb, resulting the the "rich" air/fuel mix it sounds like you are getting. You possibly left the engine set over the winter with fuel in it. The gas turns to varnish and "gums up" the needle valve so it sticks. You can try running some Gum Out--available at auto parts stores--thru it following manufacturers directions, but you will probably have to have the carb rebuilt. Good luck.