Gotta agree with Tom(?) about the NV assy or rather the total system. Only turn open sounds too tight, I would normally expect 2-2.5 turns for a "normal" setting. The other wildcard is that you have to change the setting - drastically - between starting and running. That just should not be required.

* What direction is the spray bar hole pointing? It ought to be pretty close to straight down, parallel to the axis of the venturi.

* Leaks under the venturi (ineffective gasket) or around the base of the NV assy letting air bypass the venturi could concievably lead you to open the needle for starting - but that seems a stretch.

* I think you should (and you probably already have tried this) also try removing the NV completely and pass a fine wire through the spray bar. A piece of debris could keep the needle from closing all the way and also cause erratic settings. Could also be a burr in there from the spray bar hole.

All else fails try replacing the NV assy...

Dennis A