B is the throttle barrel retaining screw. The barrel is the part of the carb that turns. There is a slot machined in the base of the barrel that causes the barrel to move side to side as the barrel rotates. It is this side to side motion that opens and closes the spray bar.

I agree that the carb looks gummed up. My advice would be remove the carb from the engine completely and then completely disassemble it. A soak in fresh fuel would probably help to loosen the gum. Take your time, work slowly, and take notes if necessary. Most carbs can be easily disassemble in a few minutes without adjusting any of the settings (low or high speed needles). This will also give you the opportunity to thoroughly inspect all the passages for blockages.

To remove the carb, there should be a screw on the engine that backs out at the base of the carb. Then loosen screw b until the barrel slides out of the carb (from the throttle arm side). The high speed needle assembly (opposite the throttle arm) should unscrew from the throttle body by turning the nut at the base of the high speed needle assembly.

When you put it back together after a good cleaning, things should turn freely without binding.

Brad