Direct intake manifold vacuum signal comes on faster and stronger than any other ported vacuum signal. Personally, I would never run a ported signal, only direct intake vacuum.
Now, PLUG or DISCONNECT the vacuum signal while you adjust the fuel/air ratio mixture screws at idle. It's just easier for MSD to tell you to use a ported signal so they don't have to tell you about adjusting the carb, both fuel/air and idle rpm. For many folks it's just to complicated. For those that have a clue, direct IS the best way. AFTER you have dialed in your air/fuel idle screws, unplug and allow vacuum signal back to the dist. Your idle rpm should now increase, due to more initial timing because of vacuum signal at idle. Now you can adjust the carb idle rpm screw.
IF the engine does not speed up when you connect or disconnect the vacuum advance at idle that means your engine cam profile at idle rpm does not provide enough vacuum signal to make the vacuum dist work. Typically MSD starts making vacuum advance at about 11 to 13 inches. Some hot rods only idle at 9 or 10 inches.
You know what? It's WAY easier to just tell you to hook it up to a ported carb vacuum signal, done, no more talk story all ready.
