I have a 1995 petrol 2.0L Tempra SW. (Yes, still going having only just passed emissions test) I'd like to increase idle rev speed a tad. On the inlet manifold, on top centre off the throttle/butterfly body is a 1/2 inch brass screw....what is it? If I turn it clockwise engine goes lumpy, anticlockwise engine goes back to normal with same low idle. Is this the idle adjustment screw or some sort of choke adjustment.