Greg, you may have there issues as the +/- I gave you are for a good tune as far as all else is concerned. Timing, sticky advance weights, float level, any kind of a vacuum leak and the carb internal seals all can cause adverse idle conditions which will render proper adjustment of the idle mix most difficult and almost impossible to get set.
What is the state of tune from above? Any recent work on the carb? Did you check for any vacuum leaks?
Can you describe what has been done and what is currently being tried?
Dave Neptune