I'm not a smog expert, BUT it is clear that when you raised the idle you went sufficiently off the idle circuit and into the cruise circuit, indicating that the problem is in the carb idle circuit and one of the big reasons that I sold my '79 late last year and replaced it with the Durango (let the computer adjust things properly - or at least better than I can). Nothing against carburetors, just against carbs that have to pass smog....and not a judgement on you for having that truck, just what works for me.