Vac Advance Unit, Testing Options

[76-914]

Is there a proper test for the dizzy vac adv unit? i.e., how many pounds vac and how long should it hold. Also, what about the retard side; the same?

[Dave_Darling]

Wow, I really thought the charts had made it into the tab at the top of the page. And I couldn't find the info you mentioned on Pelican.

So, here's a chart from Way Back When:

http://web.archive.org/web/20031213100259/...ch/distrib.html

If I recall correctly, I made all of the advance/retard figures into crankshaft degrees when I put the chart together. Also crankshaft RPM, so you can just look at the tach.

The general idea is that you have the base static timing, which is set by rotating the distributor body. Then you have the centrifugal advance, which is added to the static timing depending on engine RPM. Then you have the vacuum advance and retard, which are added to the timing after that.

To verify those figures, you would either have to set up a degree wheel or degree scale of some sort on your fan shroud, or use the advance knob on a timing light equipped with one and check what knob adjustment results in the TDC mark showing up in the timing notch.

So you could check the timing at idle, and at 1500 RPM, and at 3500 RPM, all without the vacuum advance/retard hooked up. Then you could use a vacuum pump to pull a specific level of vacuum on the advance only, and run the engine at 1500 RPM (or 3500 RPM) and see what knob setting shows the TDC mark at the timing notch. The difference between the centrifugal advance and the one with the given level of vacuum on the advance fitting is the amount of vacuum advance. Repeat at different vacuum levels, and repeat all over again with the retard only.

Kind of a pain, if you ask me...

--DD