[jhv48]

I believe you have a secondary problem. Since the carb is a vacuum secondary, they don't open up at any set throttle application. They open as demand increases.

Could be an obstruction in the secondary main jet. Did you take the carb apart and clean it prior to installation? If not, suspect tiny shaving in an orifice it doesn't belong.

Otherwise, possibly a sticking distributor advance. Easy to check with a strobe light.

If my car, replace carb with a known carb and see if problem disappears. If not, then go to the distributor and verify that it the advance is not sticking.