When your truck is warmed up, remove the air cleaner and operate the throttle to simulate condition you describe above. On two of my squares, the Qjets had a sticky choke valve, causing the engine to bog and consume a lot of gas. These choke valves and associated linkage have a tendency to carbon up and stick. I had an Edelbrock that had a choke valve that was manufactured slightly too big for the opening in the carb - I had to remove the valve and file it off slightly - then the choke would never close off completely and caused hard cold starts.
I would try a spray can of gumout to clean the carb when it is warmed up.l