Refresher Course, Please: Vacuum Advance Line Bolcked Off?

[Boomer]

Please excuse what is probably an obvious question to most folks here. Aside from my 1951 Farmall, haven't worked on anything with a carburetor muchless a vacuum advance in years. Noticed the hose was missing on my 383's distributor. Looked up under the air cleaner for the nipple on the carb and found the hose still attached, with a small screw in the end. Once upon a time I knew why this was done (testing, timing, or?) but can't recall now. The engine has several issues like not running very long before stalling, running rough, hesitation/dying when you give it the gas, etc, so I'm just starting to track down all the gremlins. The car ('66 300) appears to be a victim of neglect more than abuse. Nothing major wrong (so far) but a gazillion little things most likely from sitting too long as well as deferred maintenance.

 

[1970cat]

chances are that the vacuum advance has a bad diaphragm. test it with a vacuum pump. it is unhooked and plugged to set timing.

 

[Fratzog]

FYI here's something I saved a while back...

TIMING AND VACUUM ADVANCE 101 - Written by John Hinckley

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so
they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

The following from:
http://www.setyourtiming.com/Timing_Settings.html#VaccumAdvance

 

[Big_John]

The BIG issue with Mr. Hinkley's tech write up is that 60's and early 70's Mopars use venturi vacuum for spark advance and not manifold vacuum or ported vacuum.

It works completely different than he describes....

Venturi vacuum increases as air flows through the carb. It does not drop at WOT like manifold vacuum. It increases with acceleration, not decreases. It works the same as vacuum operated carb secondaries.

To answer the original post....

I would check the engine timing as is. Give it some gas and watch the advance. Then I would connect the vacuum, making sure you are using the correct port. Check the timing again and watch the advance again. See a difference? If not, chances are the vacuum pot is shot. There's other ways to check this, but I'm giving you the quick and dirty way.

If it works... connect it up and drive it. If all the stars are in alignment, you should have better acceleration and gas mileage. It may be that it was disconnected to keep the pinging from cheap or lower octane gas to a minimum. Some even disconnect it so they can run regular in a false attempt to save some pennies. The gas mileage decrease cancels that out usually.

 

[Fratzog]

Thanks for the correction John. You know what they say, a little knowledge can be dangerous...which is why I have a qualified mechanic tune my cars. This article may be a better reference.

https://www.google.ca/url?sa=t&rct=...o0z_KLnWX0X_GisSQ&sig2=iQtKfvZy903CEt3vJHhIzw

 

[Boomer]

And due to my lack of recent experience, I plan to do the same. Just need to keep it running long enough to get to the garage.

Thank you all very much for the great info. It definitely fired some long-dormant brain cells reading about disconnecting for timing, as well as the dreaded ping or spark knock. I recall a lot of folks changing their timing, probably bypassing the vacuum advance too, in order to use lower octane fuel. I actually remember a time when Premium was difficult to find in my area, regular was pretty much it. Between the oil 'shortages' and all new vehicles running 86-87 octane I guess demand wasn't there.

Still need to figure out the hood situation before I can dive into the engine work. Hope to get after it as soon as yard mowing is done (seems like I just did that?). Fingers crossed that the vacuum advance has just been disconnected for octane reasons, replacements don't seem to be very abundant. Will post back what I find.

 

[Boomer]

Got the hood fixed and a little time to play with the engine Sunday evening. Decided to hook up the vacuum advance and start the car, since it would stall before I ever got out of the car to do it. It started but ran FAR worse - felt like only 4 cylinders were firing. So I replugged the line and decided to pull the air cleaner to have a look at the carb. Found a bit of a surprise underneath. Stock carb is gone, no idea what issues the replacement may have as far as working properly with the rest of the systems like the vacuum advance. It's easy to assume 'drop-in' replacement, but I'm nowhere near smart enough to know that. The electrical connection in the side (choke?) isn't connected as the connector is at the back of the carb and doesn't reach.

I did discover a clear plastic fuel filter that feel soft and punky. The element inside is askew and appears to be at least partially blocking the outlet tube, so perhaps the stalling is a case of fuel starvation. A cheap starting point, at least.

 

[1970cat]

not sure what that wire was for but if it is key on 12 volts it can be used for the choke. a working choke might solve a lot of your issues. fuel filter needs to be changed as you know. the fuel inlet at the carb looks a little damp too. there is a metal crush ring in there that may not be tight enough or crushed beyond ii's usefulness. don't over tighten and break anything. interesting carb stud too

 

[twostick]

The BIG issue with Mr. Hinkley's tech write up is that 60's and early 70's Mopars use venturi vacuum for spark advance and not manifold vacuum or ported vacuum.

Care to post the source of this info?

Kevin

 

[saforwardlook]

The BIG issue with Mr. Hinkley's tech write up is that 60's and early 70's Mopars use venturi vacuum for spark advance and not manifold vacuum or ported vacuum.

It works completely different than he describes....

Venturi vacuum increases as air flows through the carb. It does not drop at WOT like manifold vacuum. It increases with acceleration, not decreases. It works the same as vacuum operated carb secondaries.

To answer the original post....

I would check the engine timing as is. Give it some gas and watch the advance. Then I would connect the vacuum, making sure you are using the correct port. Check the timing again and watch the advance again. See a difference? If not, chances are the vacuum pot is shot. There's other ways to check this, but I'm giving you the quick and dirty way.

If it works... connect it up and drive it. If all the stars are in alignment, you should have better acceleration and gas mileage. It may be that it was disconnected to keep the pinging from cheap or lower octane gas to a minimum. Some even disconnect it so they can run regular in a false attempt to save some pennies. The gas mileage decrease cancels that out usually.

It is not often that I differ with Big John, but he is not correct in this case. The article by Mr. Hinkley is generally correct (and the use of race distributors with no vacuum advance on street engines is a really bad move just as he says). The vacuum to virtually all the Chrysler distributors in the later 60s and 70s was ported, not ever venturi. There were just a couple straight manifold vacuum systems due to emission constraints as noted in the article. In fact, the only venturi ports on Mopar factory carburetors were for the exhaust gas recirculation (EGR) systems, and those needed a vacuum amplifier to boost the signal to be able to have enough vacuum to modulate the EGR valve. Venturi vacuum signals are very weak as you might expect due to the limited vacuum that could be developed from the increasing velocity of the intake air in the hour glass shaped area where fuel is introduced into the throat(s) of the carburetor. There were some models of 318/360s that used straight manifold vacuum to the distributor vacuum advance units in the late 70s and/or early 80s. I worked mainly in the fuel systems lab in the decade I spent at Chrysler Engineering in Highland Park Michigan in the late 60s and through late 70s, so I am very familiar with these systems.

What may have been confusing to Big John is that if one were to put a vacuum gauge on the end of the rubber hose at the distributor vacuum advance can, it might look like a venturi vacuum signal because as you increase the speed of the engine off idle, he might have seen the vacuum gradually rising. But this was due to the presence of an orifice spark control valve (OSAC valve we called them) mounted on the firewall that delayed the vacuum to the vacuum advance for as long as 25 seconds to help reduce hydrocarbon and nitrogen oxide emissions.

Chrysler's emission control systems were generally "retarded" (pun intended) through that time period and the cars ran like crap (lean burn was a joke). The engineers in charge at the time didn't realize how to take advantage of the newly introduced catalyst in 1975. I tried my best to get them to change their ways and go and drive and copy GM systems (they were most responsible for introducing the catalyst and knew how to calibrate the engine for good driveability and let the catalyst do the emission cleanup), which were light years ahead, but Chrysler management was too bull headed to consider something new or even go and drive a competitive vehicle (Ford wasn't much better than Chrysler either). In my view, those "stuck in the mud" ways of management were the prime reason Chrysler went bankrupt in the late 70s (and the unions) and that is why I left. But that is a very long story for another time.............

Steve

BTW, if you have one of the cars with an OSAC valve, bypass it and your engine will live a lot longer for it and get much better fuel economy and driveability.

 

[Big_John]

I don't have enough time to fully research and back up what I said this morning... Work calls and I must go....

I think the real question here is what year they changed from venturi vacuum to ported vacuum and the OSAC valve. The first OSAC valve I saw was on my 73 Barracuda that I bought in 74.

The car in the original post is of 1966 vintage and I presume it has the 1966 carb and engine. If there is not venturi vacuum used on this car, I would be very surprised.

My expertise (using that word loosely LOL) is with late 60's cars. It ends at 1970. I checked my 1970 FSM this morning and it shows vacuum for the distributor being taken above the throttle blades in the carbs that I checked. Again, a lack of time prevents me from digging into it and quoting the FSM or other sources.

I'll be back to debate this later...

 

[saforwardlook]

I don't have enough time to fully research and back up what I said this morning... Work calls and I must go....

I think the real question here is what year they changed from venturi vacuum to ported vacuum and the OSAC valve. The first OSAC valve I saw was on my 73 Barracuda that I bought in 74.

The car in the original post is of 1966 vintage and I presume it has the 1966 carb and engine. If there is not venturi vacuum used on this car, I would be very surprised.

My expertise (using that word loosely LOL) is with late 60's cars. It ends at 1970. I checked my 1970 FSM this morning and it shows vacuum for the distributor being taken above the throttle blades in the carbs that I checked. Again, a lack of time prevents me from digging into it and quoting the FSM or other sources.

I'll be back to debate this later...

OK Big John, I see where the confusion comes from - it is basically due to terminology and some misunderstanding as a result. I will explain later, but now it is my turn to run and get some things taken care of. More on this later. Not to worry!

Steve

 

[Boomer]

not sure what that wire was for but if it is key on 12 volts it can be used for the choke. a working choke might solve a lot of your issues. fuel filter needs to be changed as you know. the fuel inlet at the carb looks a little damp too. there is a metal crush ring in there that may not be tight enough or crushed beyond ii's usefulness. don't over tighten and break anything. interesting carb stud too

Got a good chuckle when I saw those Jack-and-the-Beanstalk studs sticking up. Maybe whoever installed it had bad eyesight and wanted to make sure they got it on straight?

Will check the fuel inlet to see if it's snugged up well. Not sure where to get another crush ring/washer if it's toast? Spark plug rings are pretty easy to find, but too big.

Planning(hoping) to get in another installment this weekend to change filter, extend the choke connection, and see what else I can get into. I know when the vacuum advance is hooked up it runs horribly, so that will require further testing too. Wasn't sure if the aftermarket carb or jetting had something to do with it or the diaphragm is just toast.

 

[twostick]

I checked my 1970 FSM this morning and it shows vacuum for the distributor being taken above the throttle blades in the carbs that I checked.

That is called ported vacuum. It is still manifold vacuum but because it is ported in above the throttle blades, it gets shut off if you will when the blades are at idle. Once you crack the throttle the port sees full vacuum from the manifold. Strictly an emissions thing to IIRC reduce NoX at idle which fits with your timeline.

Kevin

 

[Big_John]

OK Big John, I see where the confusion comes from - it is basically due to terminology and some misunderstanding as a result. I will explain later, but now it is my turn to run and get some things taken care of. More on this later. Not to worry!

Steve

I'm looking forward to your explanation. After doing a bit more reading, I think I may not fully understand.

 

[saforwardlook]

I don't have enough time to fully research and back up what I said this morning... Work calls and I must go....

I think the real question here is what year they changed from venturi vacuum to ported vacuum and the OSAC valve. The first OSAC valve I saw was on my 73 Barracuda that I bought in 74.

The car in the original post is of 1966 vintage and I presume it has the 1966 carb and engine. If there is not venturi vacuum used on this car, I would be very surprised.

My expertise (using that word loosely LOL) is with late 60's cars. It ends at 1970. I checked my 1970 FSM this morning and it shows vacuum for the distributor being taken above the throttle blades in the carbs that I checked. Again, a lack of time prevents me from digging into it and quoting the FSM or other sources.

I'll be back to debate this later...

Now I am back and I see that TWOSTICK has added to the conversation. He is correct, in that the small slot(s) just above the throttle plates are indeed the souce of ported vacuum and not venturi vacuum. These slots see manifold vacuum when the carburetor is off idle and almost no vacuum when the thottle is closed. Distributors that see manifold vacuum all the time normally just use a vacuum hose attached to the same multi-nipple fitting that supplies the power brake booster near the rear of the intake manifold and screwed into one of the intake runners.

The slots that provide the ported vacuum are part of the lower carburetor body bore. The venturi area is also in the throttle bore, but the venturi is above the throttle area and is where the bore necks down near where the fuel sprays from the fuel nozzle(s). The overall shape of the carburetor body bore is like an hourglass, and the venturi is the part that is necked down only. Physics requires that in order to maintain a constant flow past a necked down area, the pressure in the necked down area has to drop to speed up the airflow in that portion and then return to the previous level and speed when past the venturi. So the pressure change in this necked down area is proportional to airflow speed. Another way to say this is that venturi vacuum increases as airflow increases, and it is proportional. But these venturi pressure changes are far too low to be used to run a distibutor advance can or pretty much anything else in a car that relies on vacuum to operate. But venturi vacuum is sufficient to draw fuel out of the carburetor bowl(s). And that is partly how fuel is indeed metered to the engine.

So what you have been thinking is venturi vacuum is really manifold vacuum (ported vacuum) and very different. The behavior you ascribe to venturi vacuum signals is generally correct though, but misapplied because of the mixup between where the distributor vacuum supply slot is located and what that area is called.

But in the case of exhaust gas recirculation (EGR) valves this linear increase of vacuum is just what is wanted (like fuel flow) - as engine speed and load is increased, the amount of EGR desired should also increase. So the only time Chrysler used venturi vacuum, beside for fuel delivery, was to help operate EGR valves. But to do this, they had to amplify the vacuum signal some 10 - 14 times through use a vacuum amplifier (it was that round plastic thing at the back of the engines generally with a bunch of vacuum hoses attached to it in a coordinated fitting - their use started around 1974). Unfortunately, these vacuum amplifiers were not very durable and gave up the ghost pretty rapidly.

The OSAC valve really has little to do with this discussion, and the first version, called AVSAC was a transmission controlled valve introduced in 1971 and used also in 1972. To save cost the OSAC valve was phased in starting in 1973 I believe, and was just a tiny orifice that slowed down the vacuum signal. I just thought its effect on vacuum signal rise might have led to the confusion, but it was not the source of the mixup.

So you might have to rethink how the vacuum advance works but hopefully this helps the understanding. A lot of this stuff is easy to mix up. Only because I was immersed in it for so many years is it familiar to me.

Steve

 

[Big_John]

Ok.. I'm getting my brain wrapped around it now.

I was incorrect in calling it venturi vacuum and my understanding of ported vacuum was off in left field.

So... am I incorrect in saying that the vacuum taken above the throttle plates increases with air flow through the carb even though the manifold vacuum falls off?

 

[saforwardlook]

Ok.. I'm getting my brain wrapped around it now.

I was incorrect in calling it venturi vacuum and my understanding of ported vacuum was off in left field.

So... am I incorrect in saying that the vacuum taken above the throttle plates increases with air flow through the carb even though the manifold vacuum falls off?

That would not be correct, as ported vacuum always follows manifold vacuum only (except at idle), and as airflow increases, that means generally that manifold vacuum decreases. Only vacuum taken in the neckdown or venturi area increases due to an airflow speed increase and nowhere else in the carburetor body bore. That is why I think you need to take some time out and rethink how the distributor advance works. You will get there.

 

[Big_John]

You're right, I need to rethink this. This is a lot of years worth of not understanding it correctly.

Thank you for sending me down the right path and even more, thank you for doing it in a professional manner that I can understand.

 

[saforwardlook]

You're right, I need to rethink this. This is a lot of years worth of not understanding it correctly.

Thank you for sending me down the right path and even more, thank you for doing it in a professional manner that I can understand.

We are all in this hobby with C bodies together, and I think everyone deserves respect and patience no matter what. Helping each other is what it is all about. I get stuff wrong sometimes too. You are very welcome!

Steve