Very few q-jets will have vacuum at any rear ports in the airhorn. Most of those ports were to take in fresh/filtered air for the hot air choke. I've worked on several early 70's Old's carbs that had vacuum in the airhorn at one of the rear fittings, so it was used on some models.
In any case, make sure that you plug all vacuum fittings and correct any vacuum leaks.
Take a look in the carb at idle speed and see if any fuel is coming off the boosters (nozzle drip). If you don't have nozzle drip, and have full control of the idle fuel via the mixture screws, you do not need any addtional bypass air (in most cases). Some engines still like a lot of bypass air, for one reason or another, so adding it can help clean up the idle and further reduce how much transfer slot is exposed to gain more control of the idle fuel via the mixture screws.
Experience has taught me when to do this, over leaving it alone, so it's pretty much a case by case basis.
In most cases I do NOT use manifold vacuum to the advance at idle speed. A well chosen ported source is used instead. Quite a bit of information about this on the NET, and some folks will say that you MUST use MVA or you just don't know how to tune or what you are doing.
From my experience, having to run the initial timing clear off the scale to get the engine to want to idle well, tells me that the basic components (compression/cid/camshaft) were poorly chosen, and even more likely the carburetor does NOT have enough idle fuel capabilities at idle speed.
What I recomend to do first, is to set the carb up for the application, then do some tuning to see if the engine likes/wants MVA, or is fine with a lower base timing setting.
The real trump card in attempting to use MVA with heavily cammed engines, is that the timing falls out easily at low vacuum readings, requiring an adjustable advance or one with a really low spring tension.
Most folks, even some "experts" who debate this topic on the NET, so not even fully understand how the vacuum advance works. The ONLY difference between ported and manifold vacuum as far as the vacuum advance is concerned, is that timing is applied at idle and coasting with MVA. A well chosen ported source does EXACTLY the same thing everywhere else.
I chuckle when I read threads where folks try to indicate that a ported source continues to add timing beyond where a manifold source would fall off. Common sense would tell anyone looking at this topic, that ALL sources under the throttle plates at any give throttle angle, engine speed/load, would show the same reading if a gauge were placed on them.
An even bigger laugh comes when folks try to indicate that the advance could be applied at heavy/full throttle. When the throttle plates are on end, the reading(s) are near or at zero, or at least well below the spring tension found in any vacuum advance every produced.
The biggest laugh of all comes when we read a thread where the owner of a car switched from ported to MVA, or visa versa, and now his engine makes a TON more power at full throttle.
In any case, I ALWAYS recomend for the tuner to work with each individule set-up, to see what settings they like the best?
For most N/A engines with a decent static compression ratio and well chosen camshaft, about 8 to 14 degrees initial (base timing) is sufficient. That setting must be tested with a well heat soaked engine to make sure it doesn't "buck" the starter on hot restarts.
We then set up the mechanical curve to add about 18-22 degrees, all in by apprx 2800-3000rpm's. The curve must NOT start till just past idle speed. This is extremely important. IF any of the timing from the mechanical advance is coming in at idle speed, it typically falls out when the trans is placed in gear. This can cause dramatic drops in engine rpm's at idle speed, and is almost ALWAYS blamed on the carburetor not working correctly.
We set up the vacuum advance to add about 10-15 degrees of timing, then choose what source to apply the advance by testing to see what the engine likes/wants.
Some engines will buck and kick profusely with a LOT of timing at idle speed. Some respond well to it. It's boils down to a case by case basis on what the engine wants. The tuner should keep in mind at this point, that the ONLY difference is that he has the choice to add the timing at idle speed, and when coasting via MVA. A well located ported source adds in the same amount of timing at every other point.
One must make absolutely sure when choosing a ported source, that it is ALL IN right off idle. Many carburetors have ported sources that were designed to run EGR valves, or other emission devices. They have a much higher source location in the baseplate, and do not mimic a manifold source well enough to be used to apply the vacuum advance. Also be aware, that many q-jets have a well located ported source, but it has a bleed-off hole that drops out the vacuum as the throttle angle continues to increase.
I highly recomend to use a vacuum gauge when choosing the source for your vacuum advance, to make sure it is applying the vacuum to the can correctly under all driving conditions......Cliff
