That screw is for air flow and is usually just for the idle speed only as the high speed is fixed jets. As to the setting for the air I normally set it to 1 & 3/4 turns out from bottom and then adjust from there for the idle.
Check that you have cleaned out all the jets in the carby (I usually use some copper wire) as this will cause problems.
Regards,
Bruce
Please do not PM me asking for support. Post on the forums as it helps all members not just the individual.