Kohler Courage carb question

I have a 22hp Kohler Courage single cylinder engine. It has a Walbro LMK carb on it. There is only 2 adjustment screws on it. One is a long black plastic screw, that only turns a quarter to half around. What does this adjustment do? I've turned it all the way left and then to the right. Doesn't seem to do anything? Does this come out and maybe need cleaned?

Thanks