The basic things to do first are to loosen the gas cap, replace the air filter, fuel filter (if it has one), clean out the sediment bowl on the tank petcock (if it has one), replace the spark plug and spray a little carb cleaner in the intake of the carburetor with the engine running. Only spray short bursts of about 1/4 second every thirty or forty seconds.
Something else to do is to spray carb cleaner at the intake gaskets and around the outside of the carburetor with the engine running. If there is a change in rpm, then there's probably an air leak at that spot which is leaning out the air/fuel mix.
If none of this makes any difference then it's probably going to be time to rebuild the carburetor. You either have an excess of air, through a leak probably, or insufficient fuel flow, either from the tank to the carburetor inlet, from the inlet to the float bowl or from the float bowl to the venturi, or you have insufficient pressure inside the float bowl to allow fuel to be pulled into the venturi.
If you do end up rebuilding the carburetor, then you should use a carburetor soak can to clean it. Spray cleaner isn't very effective at getting into the little air circuits and such. You should also replace the old parts with a new carburetor kit and you should replace the fuel lines and filter and clean out the gas tank.
Let me know if you have any further questions or if you've already tried some or all of this. Thanks, PK.