Good to see this thread back up.
I spent a few hours working on the rig last month. Truck started going lean all of a sudden. It would start fine, drive a few miles, then it was just like I turned the key off - except it was idling, and would keep idling perfectly... it was just too lean to do anything else. I had to full chocke to limp it back home and within a few minutes of sitting it was all fine again.
I thought fuel pump, fuel filter, fuel cap not venting, everything that would contribute to fuel starvation - but the fuel level was always right there where it should be in the carb window. So I decided to rebuild the Flux Capacitor. I bought a master kit on RockAuto, dusted off the factory service manual, and set to it. Happy to report we are 10/10 again.
I never found a smoking gun in there that caused an AHA moment - but the exercise cured the ills and I'm happy about that.