Try slowly closing the choke by hand and see what effect that has. If you can close it all the way or almost all the way, that's indicative of a vacuum leak.
Or just cup you hand over the center carb to choke it out by hand. Pay close attention to engine idle speed as you do this. If the engine rpm rises slightly before it stalls, there is a vacuum leak. Not uncommon for these manifolds to leak under the valley pan. Using an aerosol spray to find a vacuum leak is not a good method, despite its popularity.