You said it right. The air door does create vacuum....or I guess more "technically" a low pressure area that allows atmospheric pressure to "push" fuel into the secondary venturis. That's close enough to dang vacuum for me. lol You're also correct in that the linkage does not limit the vacuum door opening. That is handled by a bendable tang on the driver's side of the secondary opening. The tang is bent to limit how far the door opens.