The Flat head one with a spring is the low speed adjustment. The other would be high speed.
The low speed is also idle, and it does slightly affect the high speed adjustment but not by much.
If the bike doesn't start with that low speed screw in all the way then you do have another issues either with not enough fuel, to much fuel or something letting in air somewere causing not enough fuel.
It isn't always the carb that goes, sometimes gaskets and the plastic manifold can break.
(take care with the low speed screw in all the way when it does start it will rev up and want to start driving itself make sure ur holding onto the bike inscase.)