I just had a smilar problem with my 76. It was a vacuum leak. I checked all the small vacuum lines and couldn't find any leaks. I replaced all of the small vacuum lines, tightened the nuts holding the carb down, tightened the nuts holding the intake to the head, and checked the diaphragm in the distributor. Still had a leak. Very frustrating. I decided to pull the carb to replace the gasket on the base. As I was disconnecting the hose that connects the EGR valve to the carb spacer, I pulled back the insulation and there it was! A small split in the hose that was not visible in my earlier checks because the insulation was covering the hose. Five minutes and a 3" piece of hose and I was back in business. Idles smooth as silk. Maybe this will help you.