Looks like an OS fp and as indicated by RCKen and jetmech is indeed your low speed mixture control. You need to understand however that these 'bleeder type carbs work exactly opposite of a double needle type carb and this is important: To lean the low speed mixture this screw is opened turned counter clockwise. To richen the low speed mixture this screw is turned clockwise. Agine this is opposite of a double needle type carb.

That other vertical screw with the spring under it is the idle stop screw which is usually unscrewed just enought to allow the throttle barrel to completely close.

John