I assume you are talking about the Carter Thermoquads. These are not really a bad carb, I've ran them for several years.

They can be a bit tricky to rebuild and there are a few a couple of tricks to it.

The problem you are describing however sounds like the secondary are opening too quickly. I think you will find that will find that you have a bad vacuum pot. There should be a vacuum pot on the passenger of the carb that is hooked to the secondary air valve. Pull the line on the pot put vacuum to it and see if it pulls the linkage, and holds vacuum.