I had an issue where the shaft from the steering column to the throttle linkage was too long, and it would not let the throttle plate close completely. Hence the idle adjustment screw at the carburetor didn't work. I had to remove the shaft, cut about 3/16" out, rethread the two ends and use a coupler and jam nuts to adjust the shaft. Problem solved.
You can test to see if this is your issue by disconnecting the arm from the throttle linkage and see if you can adjust the idle speed.
Tom Endy wrote an excellent article on cleaning the Zenith passageways, see it at
http://www.santaanitaas.org/wp-conte...p-Oct-2011.pdf.
JayJay