I vote with bbsd.
A quick, dirty, and just a little dangerous diagnostic technique is to look for manifold leaks: with the engine running at the lowest idle you can manage, open (just a little) an unlit propane torch and direct the torch-end around the intake and exhaust gaskets (and maybe the carburetor lower-body). If the idle speed picks up, you have a vacuum leak due to a bad gasket/fitting/etc.
