HRL makes a good point about checking for vacuum leaks.
While the engine is running, take a can of carburetor and choke cleaner and spray it around the base of the carburetor and any other potential area for a vacuum leak.
Spray it around the joints in the carburetor, both ends of any vacuum lines connected to the carburetor or manifold, etc.
You can also use an unlit propane bottle to do the same thing as the carb cleaner. If you pass over a vacuum leak, the engine rpm will change as the propane or carb cleaner is drawn into the intake manifold.
