Change them all!
If one does twist at or below the surface of the manifold, weld THROUGH a nut sitting on top into the twisted stud below. Prior to welding, disconnect any electronic gizmos you may have on the engine. Once you have the nut welded on, very easy to withdraw the stud.
Jerry - no offense, but I disagree on recommending stainless. Yes, it may last longer; but the friction is different, thus the tightening torque is different. A somewhat difficult "carburetor issue" is a vacuum leak at the intake caused by a loose carb to intake nut. Not saying stainless should not be used; but if used, the user needs to understand both positives, and negatives.
Jon.
