Another frequent source of vacuum leaks are the O-rings between the heads and the intake manifolds. These leaks can be detected by spraying a small squirt of starting fluid or B-12 where they join with the engine idling. An increase in RPM will pinpoint a vacuum leak at this point. New o-rings are cheap and easily installed. However, vacuum hoses are usually the culprit. I prefer the desmog, then the problem is solved forever...Jim

P.S.- also make sure all the screws are tight on the rubber connections on both ends of the carburetors.