Hi, turning the screw in the top of the carb clockwise makes it weaker but this really only adjusts the 'idle' mixture. I would suggest you unscrew the idle jet fully first and take it out and clean it with petrol or carb cleaner. From memory it has a thin slit down one side and this often gets clogged up. As you screw it in the amount of the slot exposed gets less so less fuel can get through. You can't really adjust the full range mixture because the main jets are not adjustable.
