Optimally you want ported vacuum, for a 57 that is a vacuum source under the throttle blade when the throttle blade is closed. When the blade begins to open, the vacuum port sees the vacuum as the air goes past the blade, and then it diminishes as the blade comes to wot. Not all carbs have a ported vacuum source, and while some that have a source, the vacuum may not be enough To actually operate the advance.
I ran a pair of carter yf carbs for a good many years. Those carbs had a ported vacuum source that was sufficient to operate the advance. Sometimes you can use the port on each carb to get the vacuum up, sometimes not.
When Tom Langdon convinced me to try a pair of pinto Holley Weber 5200 carbs (has to be twenty years ago now, maybe a bit longer) I discovered that the port for the vacuum was not providing sufficient vacuum to operate the advance. Tom had recommended full manifold vacuum which is what I used and it worked fine. Same thing on the 38/38 Webers I currently run so I still use full manifold vacuum from the intake manifold, even with a vertex magneto.
With a stock set up, you don’t need to disconnect the vacuum advance to set the timing because there is no vacuum acting on the advance diaphram at idle. When using full manifold vacuum, you now have to disconnect and plug the advance while you set the timing so that no vacuum is acting on the diaphram at idle while you check or make adjustments to the timing.
Hook up a vacuum gauge to the carb port once you identify if the zeniths have one, and watch the rpm and vacuum at different throttle blade openings. Here is a chart that has the specifications. I posted the sun specification chart for 56 and 57.
