1966 Plymouth Valiant /6 & Holley 1945 carb can only get to idle at 1250rpm. How to get a lower idle without dying?

You guys won't believe this. The clown I bought this from had a Holley 6145 mounted on it. Not a 1945. That'd be all good and all but it's missing one crucial detail: an idle mixture adjustment screw! It's just a filled hole instead of being there. No wonder it runs completely like ****.