Runs at FAST idle, not slow

Hi guys - This is my first post.
I recently picked up a 1963 Valiant 200 Convertible, it was the 1st car I owned but it was a Signet back then, since Im a tad older now, I went for the convertible..... It has a 225 slant six, w/ a one barrel holley. As the title of my post states, I can get it to run at fast idle, but it won't run when the idle is lowered, I even had to take the idle spring out to have the set screw in as far as possible.
The car ran fine when I bought it, and after a couple months the engine started giving us idle problems, we found an intake leak, so i replace the intake manifold gasket. No more leak. I've checked the vacumm advance and it seems ok. I've changed the points,plugs, dist cap, rotor, and wires. I drained most of the fuel out and put new in. I also had the carb rebuilt by a carb shop. I tried to adjust the timing, but there is no timing indicator to check it against so I did it as best I can.
One thing I'll mention is that the idle mixture screw on the carb doesn't do much at all.
Any help would be appreciated - Thanks