I know you said you checked for pinching and binding, but if the tube is large enough, and the rod is straight, there should not be a need for lube. With the engine off, close the throttle all the way, and note where the rod lines up in the tube (at both ends). Then advance the throttle all the way, and see again where the rod lines up. Does the rod go from one side of the tube to the other? (i.e. starts by rubbing on the right side, then moves across and ends up rubbing on the left side). If this is the case, something needs a slight adjustment; maybe the rod has a small bend in the middle, covered by the tube?