If I remember correctly, the main cause of this is an idle linkage screw adjusted too high. This is how it happens: someone wants to get a better tune and turns the idle mixture screws in to lean it up, then they turn in the idle linkage screw to speed up the idle. The engine then pulls fuel from the idle circuit and the jets; enough to almost keep the engine running. I know it sounds counter intuitive but on a warmed up engine, back off the idle linkage screw and increase the idle mixture adjustment screws (rich) enough that the engine will idle at an acceptably low rpm. If you have an automatic trans adjust it so it will idle in gear. This is something you can do in a few minutes and costs nothing.
This assumes your carb is not leaking internally. What kind of carb are you working on? Sometimes a Thermoquad will crack at the bottom of the wells; sometimes a Holley‘s gaskets will shrink if it sits too long, and other causes.