I believe that it is the link connecting the throttle shafts that is causing this. I witnessed this on a carb that I bent the link on to achieve full secondary opening. When the primaries were closed, the link was at the end of the slot and was pulling the secondaries open slightly . As the throttle was opened slightly, the secondaries closed. This is hard to explain but if you watch the linkage you will see what I'm talking about. This issue would probably only come up on a carb with the primary throttle plates closed a lot to attain a low idle speed , such as on a small stock or near stock engine. My particular case involved a 4777 650 dp on a stock 318.