1970s 76 DeVille carbureted vacuum advance

[Wallus 57]

Hey folks. I'm working on a 76 carbureted DeVille that has had some vacuum hoses replaced. It looks like they are routed correctly but there is no vacuum at the distributor at idle. So should the thermal switch be hooked up to manifold or venturi vacuum?
Thanks for any help you can provide, Wally

[The Tassie Devil(le)]

G'day Wally,

If I remember correctly, the Vacuum for the Vacuum Advance will not be available till the engine is sufficiently warm enough to allow the coolant to heat up the thermal switch to open the port.

This is done so that the correct anti-pollution stuff works.

What a lot of people do is to bypass the thermal switch and connect the Vacuum port to full manifold vacuum.

But, don't forget that when adjusting the Distributor for advance, the instructions say to plug the vacuum, adjust then reconnect said vacuum.

Bruce.


 

[James Landi]

I followed Bruce's work around with my '72 models (had two of that vintage).  The vacuum advance was "coordinated" with the speed of the car (and thermostat).  I bypassed all that and simply connected the advance to the intake manifold.  Enjoyed a much better idle, a noticeably improved response from the accelerator, and, I think, improved gas mileage. I was very proud of my work around, as I could open of "dwell adjustment hatch" on the side of the distributor and see the stator plate move in response to the vacuum advance!  As there are more anti-pollution devices on your car, I'd be concerned about damaging some component that did not exist on earlier big blocks.  Certainly worth investigating,    James

["Cadillac Kid" Greg Surfas 15364]

Waly

In 1976 the thermal vacuum switch (TVS) located on the driver's side of the front of the block behind the PS pump is for the Early Fuel Evaporation (EFE) system. It's purpose is to (when below operating temp) provide vacuum to the heat riser actuator on the passenger side exhaust manifold.
Distributor vacuum is "ported" vacuum and is provided by the carburetor when the throttles are open. Earlier 472/500 motors had a different TVS which controlled the vacuum to the distributor. but not the 76 500's.
Greg Surfas

[TJ Hopland]

Ya like Greg said how they did it over the years and sometimes within the models varied.  A 76 didn't have the speed related system on it, that was 70-72?  Even what and when or if that thermal switch near the base of the distributor varied over the years. 

Once you get past those temp and speed variables the options are manifold and ported.  Manifold is what it sounds like, a vacuum source from the manifold possibly literally a port in the intake manifold or it could be some of the lowest ports on the carb itself. Any of these will more or less be what ever the vacuum pressure is of the while engine.

Ported vacuum generally means its not constant and these ports are usually at least slightly higher up on the carb.  Most common setup for a ported signal for a distributor is the inlets are above the throttle blades so when the throttle is closed (idle) there is no vacuum signal. As soon as the throttle opens you get a similar vacuum signal to manifold.  As you get further into the non computer emissions they just got more complex with this ported stuff with passages that crossed other passages so under certain conditions they were or were not active.  It was a real mess till we got computers then it actually got easier because they just put a solenoid in the vacuum line and the computer would turn the vac on and off as needed.

Many engines seem to do well with a manifold source.  78 I think was a year that there was a dedicated port on the manifold near the distributor and a short hose and that was that, much simpler than other years.

Test to make sure your vacuum advance is working.  They have rubber innards so even a low mile original car would have likely failed by now. 

Also note that back in the day GM had dozens of models of these things that all responded differently to different amounts of vacuum and traveled a different distance.  Over the years the aftermarket (which includes one that comes in a AC box these days) has seemed to get lazy and decided that close is close enough so its unlikely that a replacement will perform anything like the original did. 

The vacuum rate doesn't seem to be that big of an issue but the travel distance can cause problems.  My observation has been they have way too much travel in em so you end up with pinging at cruise.  Typical fix for that is retard the whole distributor.  By the time you get rid of the pinging you have really killed the performance.  Fix for that is to reduce the travel of the vacuum advance back to something more factory like.

Shop manuals give the advance specs for both the mechanical and vacuum advance so get yourself a shop manual, advance timing light (or mark off more marks on your pulley), and a hand vacuum pump and see what you got.  You can get 'performance' vacuum advances that are adjustable both vacuum and travel.  You can also make your own stop out of a washer which I have found works pretty well most of the time.

Once you get the advance travel down to closer to the original specs you will be able to get the timing back to or maybe even a little past factory specs and get the performance back.  If you have been driving with a dead advance or had retarded the whole dizzy be ready for a shock the first time you step on the gas hard...  If you are in an Eldo hang onto the wheel because you may actually be able to break a tire loose.     

["Cadillac Kid" Greg Surfas 15364]

As TJ says (I think) there is a VERY WIDE range of vacuum advance curves and mechanical advance in the distributor itself. Example, early (say 1972) timing was to be set at 6 degrees initial mechanical with another 28 degrees in the distributor, and something like 25 degrees in the vacuum advance at 20 inches of vacuum.  1979 you keep the vacuum advanced hooked up for initial timing and there is only about 12 degrees mechanical in the distributor. Confusing? Yes, but all the timing both initial, mechanical and vacuum were orchestrated with the emissions systems to provide maximum performance at minimum emissions conditions. Change any parameter indiscriminately and performance,mileage and emissions go to hell.
Greg Surfas

[TJ Hopland]

Yep.  And in 1986 you could likely buy the correct part for your 76 but today its not that easy.  Basically I guess the short version is who ever is working on these things needs to have the shop manual, timing light (ideally with the advance feature), and a hand vacuum pump gauge.  You can't just throw in a replacement part and hope.

A regular timing light just flashes when it senses a spark.  The issue when you are trying to do more advanced things (pun intended) is the timing marks are usually only set up for the range of setting base timing say 5-10 range.  If you need to read something at say 20* you can't without changes.  A advance timing light either has a dial or a digital readout and +/- buttons so what you can do is 'dial in' 15 degrees into the light so now when you read 5 on the pulley you add that 15 and you are at 20. 

If you already have a decent timing light and its not an advance model you can just add marks to your pulley that do basically the same thing.  Take a piece of tape and lay it up against the timing scale so you can see the marks and make those marks on the tape.  Lets say the points were at 0,5,10.  You now move the 0 on the tape to the 10 on the scale and transfer those marks to the pulley.  You can use chalk or grease pencil if you don't want permanent marks.  It works the same as the advance light if the scale reads 5 on the 10 mark you just made you are at 15. 

You can often find timing light used because they have been pretty much useless on cars built after the mid 90's so lots of mechanics get rid of them because they never use them and even home mechanics may eventually get rid of their 'old' cars and no longer need them either.  I got a nice Snap On digital unit for $75.  I'm sure it had to be a few hundred bucks when it was new.


For the vacuum tool this is the sort of thing every car DIY person should have since it has many uses on a car and they are cheap now.  Parts stores often have their own brand sometimes even in the $5 tool bin.  This example is harbor freight and $25



     

[TJ Hopland]

Oh ya I forgot you also need a tachometer.  My fancy timing light has that too which is why I forgot.  These too often come up for sale cheap and are often called engine analyzers.  I just looked in my area and there are 2 of this exact model right now one looks new in box for $10.  If you have a car with points you really need something like this to check the dwell.



You can also get the tach feature in a modern digital volt ohm meter that has auto specific functions.   Like other things they are cheap now so any DIY person should have something and these can be used around the house and garage too.  I can't find an example of one of those at the moment and am out of time.