The problem is most likely the carburetor. It may have been mucked up by ethanol in the fuel. At a guess I'd suggest either the float isn't floating, or the float needle isn't closing correctly.
You may be able to get this cleaned up by running some SeaFoam or BG44K treatment through the gas tank. Make it a fairly high concentration of it (double what the can says for dilution in the gasoline). This won't hurt either way.
It may also be an obstruction in the intake. Assuming there is an air filter present, check it to make sure it isn't clogged up. Also check to ensure all of the air ducting is in good shape. If something like that is crushed or deformed, it could be causing a restriction, which would cause higher than normal vacuum while running, which would cause the carb to dump excess fuel.