Hi-

The first thing I would check it to remove the throttle linkage from the carburetor to make sure the throttle linkage isn't binding and not allowing the throttle blades to close completely. If they do close completely and the idle is still high, I would suspect a vacuum leak either at the carburetor or a vacuum line. Try plugging any other vacuum lines going off the carburetor and see if the problem goes away. With the engine running you can either use some WD-40 or similar lubricant spray directed at the base of the carburetor while the engine is running to fine the leak. The spray covers the leak and the idle will fall for a split second. You can also use a hose attached to a propane canister with the hose directed around the base of the carburetor..a change in idle speed will tell you if their is a leak. Are you sure you just don't have the timing too far advanced on the distibutor? If you disconnect the vacuum hose to the vacuum advance on the distibutor will the idle drop? That would tell me the throttle blades are open too far and to inspect the carburtor further.

Joe

Joe