You will still change the pressure with the set screw if the spring is for some reason .. not working the way it should. Altho I think the spring would have to be heated up alot... or really old to ever not hold a steady pressure. Aftermarket fuel regulators may leak..but they dont normally wear out.
I still thinkyou should purge the old stuff first and start from there. Old gas wont burn and will give you a rich reading... cause its not burning. The car may start but as soon as tries to idle It cant find stoich with weak gas.