CorvetteForum - Chevrolet Corvette Forum Discussion - Which quadrajet port for vacuum advance???????????????????

Chadman -
The stock routing for the vacuum advance is to use a "ported vacuum" source for the vacuum advance. A ported vacuum source produces no vacuum signal at idle, but produces normal vacuum signal once the throttle blades are opened up just a tad. The ported vacuum source on the early Q-Jets is the port on the forward, driver's side of the carb just above the idle mixture screw. All other ports on the Q-Jet are direct manifold vacuum. You had mentioned the "front left" port.... I assume you mean the port on the front passenger side (left side looking at the front of the carb.... this port is direct manifold vacuum for the auto trans modulator. The fitting sticking out of the choke area on the passenger side of the carb is also direct manifold vacuum used for emissions hose routing on vehicles equipped with thermostatic valves in the air cleaner. Either one of these can be used if you wish to run straight manifold vacuum. Ported vacuum is only available at the port on the forward driver's side, although I have seen some early carbs with no ported vacuum source.

You can change the vacuum advance signal from ported to direct manifold vacuum, but some caution is in order:

1. Some engines respond very well to direct manifold vacuum at idle, since this brings in a bunch more ignition advance. If you have a big cam, it will work really well. Other engines don't like much more than 16-or-so degrees of ignition timing at idle, and will idle rough/poorly with too much advance. If ytou engine does not like the additional advance, use the ported source.

2. It is imperative that your vacuum advance control unit (the vacuum "can) be correctly matched to your engine vacuum requirements if you use manifold vacuum. It must pull in its FULL range of advance 2" of vacuum below (lower numerical value) the vacuum you have at idle. This assures that the unit is pegged out against its stop while you're at idle, assuring that the timing, and thus the idle, remains stable. If you have a control unit that only pulls in its partial range of advance at idle, the unit will fluctuate and "hunt" at idle, making your idle erratic and unpredictable. If this happens, you must either replace the unit with a different spec unit, or use the ported vacuum source.

3. Emissions (hydrocarbons) go through the ceiling when you advance the timing with direct manifold vacuum. If you have to pass an emissions test, I suggest you run ported vacuum.

4. Benefit: Most cars will run cooler at idle and in stop-n-go traffic when you run direct manifold vacuum. But the cautions outlined above must be heeded.

For more info on this, and specs, pick up a copy of my vacuum advance paper in the Forum Tech Tips section.

[Modified by lars, 2:46 PM 9/6/2002]