If you can't prime the engine by setting for full throttle and covering the inlet with your finger, you have an air leak somewhere.

An airleak can also cause changing RPM at full throttle, as the vibration of the running engine causes the leak to allow more or less air in, changing the mixture setting.

I'm not familiar with the carb in question. However, in general, if there is a screw (or two) sticking upwards from the top of the carb, it's there to either limit the carb travel (called the throttle stop screw) or to hold the carb barrel in the carb.

The low speed mixture will be one of two things. It might be a small screw opposite the needle valve (or spraybar on RNV engines), This is the low speed needle, and the carb is a "two needle" carb. (there are actually a couple of different types of two needle carbs, but that's more detail than we need).

Or, it will be a small screw parellel to the needle (or spray bar) that opens and closes a small hole in the front of the carb. That second type is called an "air bleed" carb, and the screw is the airbleed screw.