[Excaliber]

Direct intake manifold vacuum is a good call, in fact "back in the day" it was an old drag racing trick to get the vacuum signal respone time just a little quicker (didn't really work). But be advised your "base" timing will include base + vacuum advance + mechanical advance when using this method.

Ken and Mikes's posts are right on the money!

Manufuacturers (old school stuff) used a vacuum port placed just ABOVE the lower throttle plates. That way the vacuum advance would not come into play until the throttle was opened just slightly. Avoiding potential "to fast an idle" condition due to vacuum signal advancing base timing.

At WOT (wide open throttle) there is low or no vacuum present so your running on base + mechanical advance only. You should set your base timing and FULL mechanical advance with the vacuum port plugged and the vac advance NOT active at the time.

So WHY do some engines use vacuum advance anyway? Better "cruise rpm" fuel mileage is the MAIN reason. Say your total advance (base + mechanical) doesn't come fully in until 3200 rpm, but your cruise rpm is only 2800. Under partial throttle and load conditions (such as cruise) engines GENERALLY respond better to more advance. The vacuum signal is fairly strong at cruise conditions, thus advancing the timing using the vac signal is a "good thing" in that case.

Stepping on the gas to pass or accellerate lowers the vac signal and your back to base + mechanical advance. That keeps the timing from potentially causing "ping" or "knock" under those conditions.

Many people simply ignore the vacuum advance and set up their engine for base + mechanical and let it go at that. That works fine. But again, MANY engines respond favorably to a crusie rpm vacuum signal approach to timing control.

You might want to drop the jets a hair, but better to run a little on the rich side than risk being to lean!