I'm assuming you have a Rochester carburetor, yes? No matter what you have it sounds like you're running lean. If that is the case, you should first get a vacuum gauge and see what it tells you. You may have a vacuum leak somewhere (carburetor to manifold gasket or somewhere in the intake manifold...possibly manifold to head gasket but not generally that). You're getting too much air relative to the amount of fuel you're getting. If vacuum is good (at least 15 pounds at idle and relatively steady) then I'd take the carburetor apart and clean it, check the float level (it might be set too low...which will cause it to run lean) and see what else is happening in there generally speaking. I'm certain you know this already, but the idle mixture screw will only affect the idle and not much once you get past that (or that plus off-idle).
