my brother has a 83 (i think) gmc s15 with 2.8L you start it when its warm and it revs to 4,000rpm and stays. This truck has a carb. the throttle is not sticking.

 

something is out of adjustment, most likely the idle screw

 

Look for vac. leaks as well.

 

Make sure to check the fast idle adjustment also. There might also be a vacuum mechanism that is involved in the carburetor throttle position, so you should probably check for anything controlling the carburetor. There might be one that has a vacuum switching valve that is stuck or something similar to that. I also agree with checking the idle and vacuum leaks.