[Steeveedee]

The "Q" in Quadrajet means "four", so there's no such thing as a 2BBL Q-Jet. Maybe you got a spacer for a square bore instead of a spread bore. As far as the idle and off-idle performance, the extra height could cause a lean condition, or (more likely) you may have had a small vacuum leak at one or both of the spacer gaskets. I put a thermal isolator on an Edelbrock carb I had many years ago and it leaked like a big dog. I ended up sealing it with Form-A-Gasket. I was commuting long distance in those days and working a lot of overtime, so had little time to fiddle with it.