Of Vacuum Advances and Instant Karma (or what I did over my Labor Day holiday)

[Lee]

A few months back the subject of vacuum advance was raised, and Tony asserted that no original units could possibly still be in operation, to which I replied with a picture of my still-operational 1962 original.

I learned a long time ago that if you wanted to jinx the operation of a car on a long trip, you really needed to do no more than to start bragging about how well “the old girl” was running. But who knew that also applied online?

It began subtly enough, when I noticed that the mileage log I’ve always kept took a turn from the 15-16 mpg I normally get on the highway went to 13. I thought that could just be an error on my part, until I was down to less than 12 on the next tankful. Otherwise, everything seemed normal, with about the only clue being the idle was slightly rougher.

I started troubleshooting with the basics of carburetion and ignition, and it didn’t take long to isolate the vacuum cannister. So I ordered the Standard Motors VC-25 from Rockauto, which is sold for 1958-65 models. It is not sold as being adjustable, but it is.

It bolted up with little fuss, though you do need to replace the metal vacuum line on my year with rubber hose. At the same time, I decided to purchase a dial-back timing light (thanks for the tip, Chevy!), rather than dragging out my big shop diagnostic scope. The results: not even close to ‘62 factory specs, no matter how it was adjusted. The curve was different, and it is limited to a max of 18 crankshaft degrees (factory is ca. 25*)

So I did a little research, and found that the unit sold for ‘66 and up (VC-31) IS sold as adjustable, and has some presence on Mustang and other Ford forums as the hot tip. I couldn’t find specs on it, but ordered one anyway. These units are physically identical, with one exception: the bottom screw holes are not threaded, so you need to use a couple of 8-32 nuts to engage with the mounting screws.

Result: only slightly better than the VC-25. And after much fiddling with the initial + vacuum combinations, the fact is that I can’t get the same nice long (and economical) advance at cruising speeds that I had with the original unit. All the “adjustment” does is limit the total diaphragm travel, which even at max. is still insufficient. And since it (and the VC-25) both come in sooner than the 7 inches of vacuum the factory specifies, I always got a little pinging now under medium speed light acceleration (like if you were just moving around a slower car on the highway), and therefore had to knock my initial timing back to 8* (it was 12*) because of it. Overall, the engine is noticeably “doggier” with the reduced advance.

Overall, very dissatisfied. But I did locate a vendor to rebuild my original (Terrill Machine in DeLeon, TX), so I’ll report back in 6 weeks or so on how that went. For the record, here are the results on the two units, compared to factory:

Vacuum (in.)…..Factory spec (crank *)….VC-25…….VC-31
5”"……………………NA………………………………….2*………….3*
7”…………………….1 - 7*……………………………. 3*……….. 4*
12”…………………..12 - 18*………………….……..7*…….……9*
16”…………………..17 - 23*………………………….12*……….13*
18”…………………..19 - 25*………………………….14*……….15*
20”……………………NA………………………………….16*……...17*
25”…………………..NA………………………………….18*……….18*

The VC-25 and 31 are so close, that it makes me suspect that they are the same unit, except for the previously mentioned threading.

The big takeaway for me, is that the factory unit is designed to both have more available advance, and do it within a much more narrow operating range (7 - 18 inches). After all, you really only see >20 inches when you are decelerating, so the mapping of these cans is a mystery to me.

[1Bad55Chevy]

How did you perform this test? The factory shop spec is showing 1000RPM distributor (2k engine RPM if performed on running engine) with a varying vacuum levels. In my mind the only way to pull off this test on the engine would be to apply vacuum directly to the vacuum advance via a hand held vacuum pump then reving the engine to 2k RPM and check the timing (base timing set to 0* so you are just measuring advance). I think that would be the only way to simulate the distributor testing machine.

When you are setting your timing at 12* how much does the mechanical advance add (total timing)?

I honestly don't think any of the testing would matter since the parts didn't work. They should just bolt on and work but parts are shit now a days! Hopefully that shop can get it all fixed up for you!

I am glad someone purchased one of these timing lights! They are good units at a solid price. I should start a new thread "throw away your vacuum gauge and buy a dial back timing light"!

[Lee]

You know, I almost wrote about the specifics of the test, but the post was already becoming book length…

So, to get down into the weeds of this, if you set the engine at at 2k, zero the light, and then add any specified vacuum (I did use a handheld pump), the engine speed might jump up several hundred RPM. But because of the speed increase, you also introduce a small, but unknown amount of extra centrifugal advance which can then be mistaken for advance from the vacuum can. To correct for this, you need to reduce the speed back to 2k before taking the reading (and hope that the centrifugal advance returned nicely to wherever it was when you originally set it and zeroed the light). Very fiddly.

I suspect that the factory specified the test at 1k (dist. speed), because there is a certain amount of drag being created by the point block rubbing on the cam, and that could potentially influence the the amount of actual advance you get, especially at the lower vacuum values. 2000 rpm crank speed is pretty close to cruising speed for these cars.

Because I have electronic ignition, there is no points drag to influence the test, so I found I got the same results by conducting everything at low engine rpm, keeping all tests below about 800 rpm, which is about where I start getting the mechanical advance coming in. So with no vacuum, set the idle very low, like 450rpm, zero the light, add vacuum, and take your readings. In my case, by the time I’d added a full 25” to the can, the idle was up around 700, but that doesn’t matter, because the centrifugal weights haven’t started moving. Much easier, and probably more accurate.

I haven’t completely mapped out the stock centrifugal advance, but the specs are max. 30 crank degrees at 4000 rpm, and I measure only about 14 degrees at 2000rpm (which is about 60mph for this car), which means I’m now probably only in the 37-40* neighborhood on the highway. I know from previous experience that I get best economy around 48* total (at cruising speeds).

[TonyC]

Not much help now, I know; but Ron Baker, the original guru of Lincoln restoration, once did a very concise write-up for one of his newsletters on how to tweak the total ignition timing exactly right, factoring in base, centrifugal, and vacuum readings. Unfortunately, I have long-since lost that newsletter, so I can't remember everything it had. Sure wish I could have saved them all; with both Ron and his business gone now, they would have been extremely valuable to this Forum.

---Tony

[Lee]

There were a few tech tips also published in the early days of the website. You can still access those at the internet archive:
https://web.archive.org/web/20060617115 ... handt.html

In fact, I just saw something that Chevy might be interested in…

[TonyC]

Ah, yes, I remember that. In fact, that gave me a jog of memory when it comes to M/C replacement: Not to try bottoming the piston, lest the internal seals become dislodged. I think I was getting a bit zealous with the bench-bleeding the last time I replaced the M/C a couple years ago...but I may have lucked out, because I haven't had any problems.

But there is so much more that Ron published in his mailers, which was never placed on the site. If I could wake up 33 years ago, I'd want to advise him to scan those mailers to the site, or at least rewrite the texts. He'd probably think I'm a loon, talking about scans, web sites, et al; but...

---Tony

[1Bad55Chevy]

Whatever happened with this?

[Lee]

They told me 6 weeks on the canister rebuild, so hopefully by the end of October. I also got some lighter advance springs, so I may play around a bit with that as well when I get to that point.

It's been running OK in the interim...we've taken a couple 100 - 200 mile day trips since I replaced it.

[TonyC]

Have you kept a tab on the miles? The mileage challenge is still open, so flaunt them.

---Tony

[Lee]

So to button this thread up…I’ve actually had the rebuilt vacuum advance back since early December, but with Christmas preparations, I decided to wait.

As earlier mentioned, the rebuild was done by Terrill Machine in DeLeon TX, and frankly, I had let it slip my mind, and when I noticed that the promised 6 weeks had become 9 weeks, I sent an email to inquire. The owner was very apologetic, and he did then ship it within 2 days. The work was well done, no leakage, but I did disassemble it to make sure all the springs and shims were in place.

After re-assembly, I mapped the advance like I did with the two aftermarket units. The numbers tell the story:

Vacuum (in.)…..Factory spec (crank *)….VC-25…….VC-31..Rebuilt Original**
5”"……………………NA………………………………….2*………….3*……..1*
7”…………………….1 - 7*……………………………. 3*……….. 4*………5*
12”…………………..12 - 18*………………….……..7*…….……9*…….19*
16”…………………..17 - 23*………………………….12*……….13*……25*
18”…………………..19 - 25*………………………….14*……….15*…..27*
20”……………………NA………………………………….16*……...17*…..28*
25”…………………..NA………………………………….18*……….18*…..28*

**with only one installed shim

These numbers are considerably higher and come in faster than either replacement, but as you notice the readings are about 3 degrees higher than the factory specification. Many years ago, I played around with the shim pack, and found that (for my driving habits, maybe not yours) more vacuum advance gave me the best results for drivability and mileage. They originally had 3 shims, which is now only one for mine.

Along with 10* initial, and another 10* centrifugal coming in by 2200 - 2400rpm, I should have around 48* total lead at 60 - 70mph cruising speed, which is right where I wanted to be.

Terrill charges a $100 flat fee for any advance canister, and that’s not exactly cheap, but the aftermarket replacements are so far off from ‘62 specs that I think it justifies the expense.