While idling (which produced natural manifold vacuum) carefully spray carb cleaner around the base of the SC. If the idle changes pitch, you have found your leak.
To change it on a Series 2, you must remove:
3 electrical connectors
several 10mm bolts
several vacuum connections.
You need a new gasket and two o-rings to replace it (if you find that you have a bad gasket which is RARE), and I also suggest replacing your fuel injector o-rings at the same time.