On a '78 EC, 350 w/Q-Jet, from where does the secondary (rear) vacuum break (choke pull-off) get its vacuum? Does it come from the manifold or the EGR TVS? My problem is that in cold weather, the car starts and then dies immediately. I go through this two or three times and then I can keep the car running if I feather the throttle for a minute or so. I know the EGR valve is not opening because the EGR TVS is closed and there is no vacuum at the EGR valve. In watching the carb during start-up, the choke butterfly is pulled open by the rear vacuum break just as the engine stalls. Moving the vacuum source for the rear break to the EGR supply after the EGR TVS would bring the rear break on after the engine has warmed up. According to the book, the the secondary break is to prevent overly-long high idle periods. That sounds to me like it needs vacuum only after the engine has warmed up.

You may want to try an adjustment on the pulloff. I don't remember how the Qjet adjusts, usually it involves bending a part in the linkage. Sometimes there is a tab you bend, other times a U shape in the linkage can be shaped to make the change. If the choke blade is opened, for instance 3/8 in. when the engine starts, try changing it to 1/4. It don't sound like much but can a difference.