Blocked fuel return line?

Just got a '79 245DL. It starts up and runs at higher RPM's, but will die if the engine drops down to idle speed. It smokes and smells like it is running very rich. What are the best things/ways to check if it is getting too much gas? Perhaps a blocked return line? Any ideas would be great. Thanks.
srockwel@willamette.edu

Probably the K-Jet mixture needs to be adjusted, done w/ an allen wrench, anyone have the specific procedure? Don't recall it off the top of my head