Why use intake vacuum vs. carb ported vacuum?
Intake vacuum source gives more advance at idle.This can help off idle response on modified engines.The the extra advance also speeds up the idle,to compensate,you close down the throttle opening,this helps some engines that run on a few seconds after the ignition is shut off.Extra advance can also make the engine run a bit cooler at very low speeds if this is problem.
On some carburetors the ported vacuum can recieve a false vacuum signal at high speeds and bring on the vacuum advance when you don't want the extra advance.
Be aware if running a very small carb the intake vacuum might rise up enough near maximum RPM's to start the vacuum advance.This is something you may want to check out with a vacuum gauge during actual running conditions.
And of course,every tuned engine is different,run yours the way it wants to be run.........
