Helping a friend do a tune up on a basically stock 350 SBC. He bought a new distributor with an adjustable vacuum advance. I've never dealt with an adjustable vac. advance before. How should they be properly adjusted?
with vacuum disconnected, set timing for around 34 degrees at 2500 rpm or so. Get the idle happy by adjusting the carb. Then connect the vacuum advance, and see how it runs and drives. If it pings at light throttle application, you might have to dial back the vacuum advance a bit. If not, then it's probably ok as is. You can check the action of the vacuum advance with a timing light. There are specs for different engines for mechanical and vacuum advance, in the factory shop manuals for all the cars that the 350 came in originally...you'll find they vary somewhat, usually due to calibration for performance and emissions for that particular year and model.
Follow the instructions, test drive and repeat. It may take multiple tests to got it right on. Be patient and see what your engine wants.
Crane (who makes adjustable vacuum canisters) suggests starting out tuning the vacuum advance with the can fully pegged to the high side and making a test drive. Bring the 3/32" with you. It will most likely ping like crazy. Back off 2 turns at a time, till engine no longer rattles. Note that excessive vacuum advance will cause a ping under completely different conditions than excessive distributor based advance. The engine knock caused by excessive vacuum advance will actually go away with acceleration, and then return when the load levels off. The takeaway though is once you get the base distributor timing and centrifugal curve squared away, don't mess with it anymore, and adjust the vacuum advance last, the tendency though is to try to compensate for excessive vacuum advance by retarding the distributor timing. This is the wrong way to go about it.
I would think experimenting with ported vs. direct manifold vacuum would also be something to think about.
I read somewhere on this site a guy installed two vacuum gauges, one to ported, the other on manifold vacuum. He drove around and both gauges read just the same, except at idle. So if you have idle issue it can help or hurt depending on where you hook the distributor to, other wise it doesn't matter once the engine is off idle. Here is a pretty detailed list of vacuum advance units, http://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdf , You can use this as baseline for the engine you are working on, note the starting vacuum and total degrees.
A general rule of thumb is that street driven performance engines usually with a radical camshaft benefit from a direct manifold connection. It is difficult sometimes to achieve a steady idle, which incidentally is why a ported vacuum distributor connection was invented in the first place. For stock or stock-ish engines, and people who don't want to make a deep dive into distributors and timing curves or custom vacuum canisters are probably better off using the factory connection.
If you want to step it up, get yourself a vacuum meter and place it, so you can see it while driving. Hook up a MityVac (or any other hand vacuum pump) to your dizzy and take her for a spin. Let say you dive at 2500 rpm. Have a friend pumping the mityvac till you read the highest (manifold) vacuum - take note how much vacuum it took. Rinse and repeat with other rpms. At the end you got yourself a vacuum curve tailored to your engine. Now find a vacuum can with a curve that is as close as possible. Frank
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