Spray some carb cleaner lightly in little bursts around the vacuum lines and base of the carb while idling. Don't use too much or start a fire... Spray a small test shot into the carb to get an idea of how it responds the extra fuel. Should surge up a little.
I have actually seen some Holley carbs have vacuum passages that are open on the bottom and not sealed by all gasket / intake manifold combinations. I did one where I had to add a heat shield between the carb and intake to get a proper surface to seal against. Also make sure any unused hose connections on the manifold are plugged.
Make sure the power brake vacuum port on the back and the two ports on the front of the carb itself are plugged if they are not used. The vacuum ports are shown in the attached picture. I highlighted the passages in red that I have seen that sometimes do not line up well with certain intake manifolds and cause vacuum leaks between the carb and manifold.