Frank,
I usually don't like to comment on something that I haven't tried. I have the same problem with my 56 TR3, won't idle below 1,500, but I can push the linkage under the hood back to an 800 idle. In a thread on BCF, I read that the universals on the linkage at front of the front carb tend to bind a bit and can cause the throttle shafts to leak (what Ken said) or just bind at the bellcrank and not let the throttle plates go back to idle (what Tush said). The recommended fix, that I have not tried, was to get 1/2 turn more compression on the two springs (just in front of each carb body) on the throttle shafts. I have not tried this yet, but looked at the springs and retainers today. I don't think I can see where the spring tabs will be held if you go 1/2 turn tighter. I am only suggesting that you look because it may be a week or two before I can try this remedy myself.
Good luck!