Which particular carburetor do you have? It's stamped on the carb body. That will make a difference. It sounds like you need the eccentric screw that goes into the steel yoke on the side of the carb.

You need to get the full gasket set for your engine to get the rear cover gasket. For all FS-120 Surpass, except the Surpass III, you need:

OSMG6197 45514010 GASKET SET 120SUR $5.49
OSMG6730 27881330 MIX CTRL SCREW 120SUR $3.09


These parts can be ordered through any hobby retailer who sells O.S. Max engines and accessories.