First off, I guess you have a BING carburator? This carburator has only one place that will supply fuel to the engine. This is the main jet. Whether it is idling or WOT the fuel is supplied through the main jet. The idle screw should probably be called a slide adjustment screw because all it does is raise and lower the throttle slide, in the Ventura, controlling the incoming air at idle. Ventura is the name of the bore through the carb.
Now the fuel fills up in the float bowl to a specific level and then the fuel inlet needle stops the flow of fuel. As the air passes through the ventura it pulls the fuel, up through the main jet, into the needle valve and the fuel mixes with the air going into the engine. If the fuel level is low, not much fuel gets pulled up. If it is high, a greater amount of fuel gets pulled in. Why? The difference in the distance that the air has to pull the fuel up.
The needle rod is tapered and hangs in the needle jet above the main jet. This tapered rod controls how much fuel passes to the engine at various throttle positions. If you look at the needle jet you will see that there is a orifice at the top that the needle drops into, it is similar to the jet but bigger.
Now the four grooves at the top of the needle rod, they are for setting the distance that the tapered part of the needle hangs in the orifice. IF the top groove is used, the tapered rod hangs deeper into this orifice, smaller opening, less fuel supply. If the bottom groove is used, it raises the rod and a narrower part of the taper is in the orifice creating a bigger opening allowing more fuel throughout the throttle positions. At wide open throttle, it is completely out of this orifice to supply the maximum amount of fuel. Hope this helps,