A number of members I have noticed are having problems at different times that need help with tuning and various other engine malfunctions that could be diagnosed with a vacuum gauge and I found this website I think will help.Scenario 14 deals with clogged exhaust or a bad cat and there are others for carb tuning,bad valves and so on,check it out I think you'll find it interesting.
http://www.secondchancegarage.com/public/186.cfm