Ok... Here is where I am on this problem. I replaced the distributor (used one on EBay) and the vaccum advance is now working correctly. I replaced the carb (used one from a supposed running vehicle on Ebay) with no improvement. I have checked and rechecked all the vaccum hoses. Sprayed wd-40 on all of the hose connections when the truck is running and can not find any leaks. I can not believe that both carbs would have the same problem so am trying to look at what I missed or other causes.
Symptom: Basically the same since I started.. that is the truck starts and idles just fine. If I remove distributor vaccum line it idles down meaning to me that the distributor vaccum advance is now working. If I accelerate slowly I get to about half pedal before it "starves" or hits a flat spot and it appears the gas in not being sucked into the untake manifold but "mists back up" through the carb. I interpret that as loosing vaccum. Not quite the same as before I replaced the distributor vacuum advnce but similar. I have put a hose and a clear fuel filter between the fuel pump and carb to make sure gas supply was getting to the carb and it seems to be OK. If I hold a vacuum hose taken from manifold the vacuum seems to interrupt or stop when hitting the "flat spot" but am not sure which is happening first (loss of vaccum or loss electrical firing or something else). I can run the truck by being gentle on the accelerator and can if fact get to 55 or 60 in high gear but it is a slow process as I have to shift around the time the "flat spot" starts.
I am wondering if the ECU could do this as it is in the middle of all the "closed loop" system according to a diagram in the manual I am using. I keep going back to the fact that whatever happened seemed to have happened after sitting overnight as it ran it just fine getting home the day before. Also, could the cat converter cause a symptom like this?? I took the Oxygen sensor out of the manifold to see if t would run better (relieve back pressure if cat is plugged?) and it didn't seem to make any difference.

Any thoughts or suggestions would be greatly appreciated??
I have not dropped the gas tank (JDK's suggestion) yet as I want to find this problem first.

 

Any luck with this problem? I have a '92 B2200 that has been doing pretty much the exact same thing for some time now. I've been leaning towards trying a new/used carb but have not bit the bullet yet.