86 K3500 GMC running very rough

[Roger2561]

Hi folks, I'm new to this forum so please be patient with me. I have a 1986 K3500 GMC 1 Ton with a 350 V-8, Dually, 4X4, 4 speed manual tranny; it runs extremely rough. When first started it runs okay, not great, but okay. As the engine warms it acts like it's going to stall out like a flooded engine. I wanted to clear it out so I thottled it, observing no telltale sign of flooding, like black smoke. Someone told that it may be in the vacuum hoses, check valves, etc... anything to do with the emissions control system. Can someone tell me which valve will most likely cause this problem. It ran smooth and quiet until this happened. Any advise will be appreciated. Thanks, Roger