Your engine would have to have to be so far out of tune for that to happen. To have an engine actually IDLE with the Main Circuit on would require the blades to be open so far that the idle speed would probably be 3000rpm! Even with a set of air bleeds that brings that circuit on at it's absolute earliest point.

I've messed with carbs for a long time, and tuned them with A/F meters and such. I know what I'm talking about here.