rough idle when tank is full?

[DRobb's_85]

My 85 k10 has a strange problem. Every time I fill the tank up, it will run rough on idle until I drive it down to 3/4 tank, where it then smooths out. It runs great from then on, but that first 1/4 tank of fuel causes it to run rough at idle speeds only, for some reason? It has a new 350/edelbrock carb in it with 650 miles on them, the fuel pump is new, and I am running a fuel filter. This just stumps me. Any idea of what it could be? For now I guess I'll just fill it 3/4 full until I can get this figured out.