Vacuum Spring Problem?

What that tells me is go back to the unpainted spring and try it. The secondaries are starting to open, then the airflo stalls (too big a carb w/4bbls open) and the purple spring does it's job and closes the secondaries. As I said, you may find the next stiffer spring from natural wil be the best, but one step at a time. The check ball allows the secondaries to open smoothly. When it's removed, there is a lean situation that can develop because there is not enough airflow right away to draw fuel out the secondary venturies, and the 30cc acc pump in the primaries isnt enough to cover it. it's an "old school" thing. It might work on a larger V8, it wont help on a 273. What rpm are you at when it studders like that?