I would turn this screw left until the engine changed it’s fine then I would turn it right and do the same and put the screw in the middle. When you are adjusting those screws, do it a fraction (1/8th) of a turn at a time. Rev the machine up, let it return to idle.....the effect of mixture adjustment isn't always immediate. This is especially necessary with the high speed run screw - as you accellerate the engine, it should buzz and then start to slightly 'four stroke' as it reaches maximum RPM. Some people think the engine is misfiring - it isn't.

The carb fitted has two adjustment screws of which are circular with a little pip in one place so the tool can only fit in one place. By all means buy a new carb if you think fit - it may be great, it may not but I would be inclined to clean the original up inside before doing so, and replacing the gaskets/diaphragms within. When the machine is running, keep it idling as fast as possible (without engaging the clutch etc). These tiny engines have little 'flywheel' effect to keep it turning over. I don’t understand why I was having to keep doing it so I thought maybe another carb would be the answer ? This then would run fine but after say 5 minutes it would start chugging/ big down etc and I would need to do it again.Could you take a look at these photos of a carb and advise the possibility of adjusting mixture or is it fixed etc without adjustment. From your description of the carb screws, it would seem you are using the 'pacman' adjusting screwdriver.