The screw adjusts mixture at idle. Push flap lightly (to simulate airflow) while cranking to see if you get it started that way.

Re mixture - You need to measure control pressures. Best take it to a garage that has the equipment to do it or you could buy a K-Jet kit off ebay or similar and have a go yourself. That way you could use the kit again if needed etc etc.

If you have a go yourself take a good while to digest how the system works so you don't make it worse than it is now.