The two-port thermo-valve activates the Auxiliary Accelerator Pump when the engine is cold (less than 120*F). This supposedly improves drivability, but I haven't noticed any adverse effect after capping the manifold and carb ports long ago. With the Mikuni, the four-port thermo-valve regulates vacuum supplied to the choke breaker and opener, canister purge valve, and the EGR valve. With a Weber the valves are just plugs in the manifold holes. Post #2 of this thread has pics of both the two and four port valves:
http://www.mightyram50.net/vbulletin...257-EGR-system