Poor compression in one cylander at idle.

Take a quart of dexron tranny fluid and put half in crankcase an other half down carb at high idle to keep it from stalling. If there is alot of gunk in crankcase it could be bad and clog up screen on pickup so be aware of this. Do the carb in open area cause it will blow alot of white smoke. We use to do this at Dodge dealer in the 70's on used cars to make them run a little better before they went on used car lot. I would say stuck or broken rings.