Vacuum advance units

[Bill usn-1]

Just to add to the distr discussion on "tuning" them for a carb.

Sticking with my desire to provide as much real world no BS info for you guys, I spent a little more time checking vacuum advance units.

A discussion has gone on for a while that the 3/5/7 could referred to the amount of vacuum required to cause the advance plate to move or the actual degrees it applied.

My thought was always the latter. After the earlier discussion I pulled one of each from my pile of goodies and here is what I found.

This is the amount of movement of the base plate with full vacuum applied.

3 = .027"
5 = .057"
7 = .087"

So as we can see, the higher the number, the more advance is applied.

If sticking with the IH distr specification chart, you should double the number to give the actual degrees applied.

So:

3 = 6*
5 = 10*
7 = 14"

But since I don't have any carbureted vehicles I can go start one up and connect my vacuum brake bleeder with the timing light connected to verify.

When tuning your advance curve this info is good to know as you work your total timing curve.

If the mech adv applies 20* (I don't think IH even has that much) then at wide open throttle that is all you will get with a base timing of 0*.
Most engines run best between 30-34* timing at WOT.
This is why our engines tend to run much better with a base timing of ~10*btdc.
10base + 20mech adv = 30*btdc
Keep in mind there is no vacuum adv at WOT.

[Eric VanBuren]

A little filing of the notch that limits the movement can turn a 3 into a 5 or 7 or even more at least on the Holley units.

[Bill usn-1]

The prestolites are pretty easy to get to but IIRC the Holleys are more internal.
I'll have to look again in the morning and get pics of each.

[Bill usn-1]

Well, it looks like it will take a small cutoff disc on a dremel to get to the slot in the arm to increase it.

Here's the Holley.
The silver is a 5 and the gold a 7.







Here is the Prestolite. Judging by the slot this has very little advance.




The one thing avail on the Prestolite is the amount of vacuum required to apply the advance is also adjustable with shims/washers between the cap and the spring to set the preload.