TheNumberOneStunner

I've got a '79 4WD station wagon with an EA71 1600 growling under the hood. We're on carb #4 I believe. It'll run great for a few days then it'll start chugging and backfiring out the carb really badly until it won't run especially after a good 5 mile drive at freeway speeds. This same problem has happened in varying degrees of badness with 3 other carbs. We've re-torqued the manifold bolts down, checked vacuum lines over and over, cleaned and re-cleaned, swapped jets, and have more than taxed todd's patience asking what in hells half acre could be the problem. I'm starting to get really frustrated. Anyone had the same problem and FIXED it. Anyone know what's wrong...PLEASE HELP !!! Any help would be greatly appreciated. Thanks guys.