Sure sounds like a vacuum problem.
Edelbrock suggests this:
1. Check for air leaks. Make sure carb and manifold gaskets are sealed properly. Make sure all unused ports are plugged.
2. Ceck ignition system.
3. Check choke adjustment.
4. Check carb fuel filter and/or in-line fuel filter.
5. Check float drop level.
6. Check for dirt in needles and seats.
7. Increase accelerator pump stroke.
8. Check float for leaks.
9. Check airhorn gasket.
10. Fuel pressure too low.
11. Check secondary latching device and rod for proper movement.


I know some doesn't apply to your situation but, these are from thr troubleshooting chart from a Edelbrock Owners Manual for 1403,1404,1405,1406,1407,1409,1410,1411,1412, and 1413.