Vacume source

[Bored&Stroked]

Not trying to be a pain in the rear, but that has nothing to do with the original question about having the correct vacuum signal to run the stock distributor. In a vacuum only distributor designed for ported vacuum (actually any given vacuum signal), a specific engine combo and a single carb (49-53 flathead) - the system is tuned to provide the right advance at the right vacuum/load condition. If you start changing any of the system components (manifold, number of carbs, cam, etc) - then the original distributor and advance/retard curve doesn't work as designed. That is why dang near everybody on this site who has hot-rodded a 49-53 engine (dual carbs, cam change, etc) - recommends that you change the distributor to one that has at least mechanical advance (RPM driven) and additionally a tunable vacuum advance (great for mileage, etc).

The main reason why many factory tri-power setups have vacuum operated outer carbs is not related to keeping the vacuum signal high for something like a distributor. The primary reason is for fuel economy - in that the vacuum aspect of bringing in the outer carbs (or in the case of a 4-barrel - the secondaries) is so that they have a load-sensing tip-in, versus a straight mechanical linkage tip-in.

Regardless of the number of carbs, how they operate (mechanical or vacuum), it is critical that any vacuum advance/retard system be tuned to the specific system it is used on. You can't expect the same vacuum signal at the same RPMs and associated load conditions to apply across all the different manifold/carb configurations - they vary greatly. Also, you cannot "tune the curve" in most vacuum systems - you can't change how much advance and/or how it comes in (unless it is computer driven). Even with a modern Accel type vacuum cannister on a GM distributor, you have limited abilities to tune it (no concept of advance bushings, advance weights, advance springs, etc).

Okay - enough of my babble . . . I'm starting to bore even myself!