In order to prevent the fluctuations in idle when connecting the vacuum advance, you either need to use ported, or, if using manifold vacuum, it needs to be FULLY engaged when idling. You can adjust the canister to where it will be fully engaged when at idle. My canister operates in a 5 inch range, from 12"-17" with 18" of manifold vacuum, therefore, at idle, it is fully engaged.
Also, the debate about ported vs manifold, it really just depends where it idles best at. Typically, you want the carb to have the correct amount of the transfer slot showing to improve off-idle driveability. For most street engines, manifold vacuum raises the idle speed too high, so you close the throttle plates and thus cover the transfer slots. With it connected to ported, the idle speed may be more ideal with the correct amount of slots exposed. An exception is when the engine's cam is very lopey causing idle speed to be too low, thus you have to open the throttle more, exposing too much transfer slot. This is when you would use manifold vacuum.
