TR-6 runs fine but seems a bit rich according to plug color. When I try to adjust mixture, nothing happens. Manual says when you lift the piston 1/4", idle should speed up and then die off. When I fooled with SUs back in the '50s, that's the way we tuned them, but these Stromberg's don't make any difference what I do, even when I screw the needles all the way in or back them way out. The idle speed just falls off right away, indicating too lean. Any ideas?