1969 2bb 350 2 speed auto. I am having a number of minor problems that have the leanings of a vacuum problem. My shop manual is of no value and Chiltons has vacuum schematics for only a dual action distributor using a 5-port thermostatic vacuum switch. Trouble is, my distributor is not dual acting and there is an adapter that plugs onto the 5-port switch which seals off 2 of the ports and converts it to a 3-port switch. I recall some years ago coming across the explanation for this set-up but for the life of me, I can no longer find any reference whatsoever to it. Help? Thanks for any insight.