You may have a sticky choke. It could also be that your choke thermostat is getting weaker with age and running the truck richer.First take some carb cleaner and give all the linkages on the sides of the carb a good bath. Maybe they are sticking. Than after the truck is completly warmed up check to see if the choke is wide open like it should be. If it is partly closed you may have to adjust it. How does the truck run when cold? I guess I should have asked what year you have . It will make a big differance.
I am sorry .i found your post during a search for new post that day. I thought you had a automatic choke and that was the problem. I will try to pay more attention to what car or truck owner I am talking to. Sorry to have butted in . I am not with the 1 barrel. On a Motorcraft 2 barrel they have a power valve screwed into the bottom of the bowl. When that goes bad it can bleed a little gas in to the engine all the time. At idle this sometimes shows up as black smoke. Going down the road the engine just burns it and the only thing you can see than is reduced gas milage.