low idle

hi..i own a 93 plymouth duster....i have an idle issue...when sitting idle with the car in drive it shakes/and i notice that the tach sits at or around 450rpm's ...ive replaced the idle control,new plugs,air cleaner and valve cover gaskets...and no,there isn't any way of adjusting the idle (the screw,the way its made will not allow you adjust,its welded in