That's not an adjustment, it would be a jet or metering device. It probably does unscrew, but you need to use a properly-fitting screwdriver to loosen it so you don't bugger up the device. You probably can loosen and remove it, but be careful to not mess it up.
The idle mixture adjustment is underneath a brass cap pushed into one of the sides of the carb.
The idle mixture adjustment is underneath a brass cap pushed into one of the sides of the carb.