[dmjlambert]

The hissing is probably it just drawing in air, which is is supposed to do. When the engine is warmed up, the smaller top choke plate should be fully open and the big one closed. If you cover it to block some of the air from going in, you are changing the air to fuel mixture. The mix should be set properly for good idle, and that is a carb adjustment. So, if you are covering the top of the carb or partially closing the choke plate and it runs better, that is because the engine is not warmed up yet, or the carb is set to run too lean or it is gunked up inside again already, or you have a vacuum leak somewhere that if you fixed it would richen the mixture. A vacuum leak can be around the edges of the carburetor where it mounts to the engine, or in a vacuum device that you have hooked up, or an unplugged vacuum port that you are not using. How many ports do you have on that carburetor, and are the unused ones plugged?

It makes a difference which port you use for the distributor. There is ported vacuum, which has no vacuum at idle, and manifold vacuum, which has vacuum at idle. If your distributor is connected to ported vacuum and then you change it to manifold vacuum port, it will idle faster and probably smoother. The factory put the distributor on ported vacuum to retard the timing and make it more fully burn the fuel when sitting at stop lights, to reduce pollution. Move it to a manifold vacuum port and see how it performs. You may need to turn down the idle speed after doing that.