'74 450slc fast idle problem

Hi, We just bought a non-running 450slc. It turned out that only the points were fried. We replaced the points, set the dwell and vroom vroom!! BUT, it idles about 1800rpm. The timing is ok and the warm idle screw is in all but about 1/2 turn. It idles faster as I turn it out. Could the cold idle valve be stuck? How can I test that part? Any other ideas? Thanks.