What carburetor do you have on it? If it is not a spread bore, it requires a thin metal plate to seal off the wider secondary side. If you do not have that, you have a vacuum leak.
I have a Holley 790 on it. It's a dual feed. Non double pumper. Wouldn't the rpm rise when I sprayed carb cleaner around the carburetor base if I had a vacuum leak? I tried that around the base and the entire intake and nothing happened