Assuming there are no internal motor problems, no leaks (vacuum) between the carb and manifold and manifold and head and the ignition including plugs and wires, points etc are in good shape it is likely carburetation. Check the plugs. If they are very black and or wet, it is too rich. If very light or if the plugs are showing wear or even metal splatter too lean.
Just a place to start. Diagnosis over the net is tough to do. Good luck!
