Need experienced opinion - Can vacuum advance cause pinging?

[Newkidonthebloc]

Hello everyone,

A quick disclaimer: I am completely new to the hobby. Everything I do is my first time. I try to do everything by the book and correct but a lack of experience and being a numbers guy probably makes me too cautious and prevents me from driving the car as much as I would like too.

Today I checked the timing (with out vacuum advance hooked up). There is quite some play in the timing chain but the car idles around 10 degrees and at 2500 rpm the timing is roughly 34 degrees. I'm fine being on the safe side of it. Engine is old/ slightly tired and the car is just a driver (for now). I am not looking for performance but I am scared for predetonation ruining my engine. It is a 383 2-barrel that now has a 4-barrel, intake and headers but I believe the rest to be stock.

So.. here is my problem: When I drive around 30-40 mph and let off the gas or touching the gas just slightly to maintain the speed I hear a faint sound that resembles (what I think) ignition. The sound is very constant (every cylinder seems to produce it) and I am worried that it might be predetonation happening. The engine sounds fine when I step on the gas or drive with more throttle engaged.

I am thinking that this might be caused by the vacuum advance since it only happens with out / or with slight throttle. However, reading about timing has taught me that pinging usually checked during acceleration or with a fair amount of load on the engine.

Driving around with the vacuum advance disconnected made the car run worse but it did get rid of this sound...

Could the vacuum advance be the problem and/or is it normal to hear this?

I'd love to hear what the more experienced think about this.

Best regards and thanks in advance (pun intended ;)

 

[AR67GTX]

It (vacuum advance) can contribute to pinging but usually when cruising at high speed at steady throttle (vacuum high and engine speed high enough to bring in most mech advance). Kind of doubt that is what you are experiencing unless someone has put real, real light advance springs in your distributor so that it fully advances by 30 - 40 mph. Try plugging the vacuum advance line and see if it quits. Could be some valve train noise but just a wild guess.

 

[copper67sat]

My 67 Satellite 383 4bbl / 727 / 3.23SG would ping under light throttle cruising in 3000 rpm range when I first got it with about 95k miles on it. It would go away under acceleration with enough gas. Running it on 93 octane Premium helped. Retarding the timing a few degrees also helped. I also played around with octane boosters - they worked, but were economical for everday driving at the time. Rebuilding the engine with slightly lower CR and a different cam cured it completely.

EDIT: not economical

 

[Thrashingcows]

Are you sure you have the vacuum advance hooked up to a ported vacuum source on the carb? I've had over heating and performance issues when I accidentally hooked up the vacuum advance to a straight vacuum source off the carb.

Do you have a dial back digital timing light and a vacuum gauge? Best way to tune the carb and motor for maximum efficiency and performance.

 

[khryslerkid]

High mileage, might have a bit of carbon build up in the cylinders/heads. Pull some spark plugs to see what they look like.

 

[Don Frelier]

Your vacuum advance canister (where the hose plugs into the distributor) should be adjustable.
You can put a small allen wrench through the port where the hose connects and try going counter clockwise on the spring setting.
This changes makes the advance mechanism requre more vacuum.
Count your turns so you can always go back.
For some light reading:

How To Limit and Adjust Chrysler Vacuum Advance Cans

 

[4406bbl]

With today's fuel you will probably need to limit the amount of advance, and tighten the spring to bring it in later. It will take a little work/ trial and error to get it right but it really helps your fuel mileage vs eliminating vacuum advance. I would shoot for 10-12 degrees total at your hiway cruise rpm and go from there, I may need less depending on compression, fuel, and air temp. I could never get the 56 degrees total on a stock bb mopar that some claim 50 was about it with normal fuel.

 

[GMP440]

Hello everyone,

A quick disclaimer: I am completely new to the hobby. Everything I do is my first time. I try to do everything by the book and correct but a lack of experience and being a numbers guy probably makes me too cautious and prevents me from driving the car as much as I would like too.

Today I checked the timing (with out vacuum advance hooked up). There is quite some play in the timing chain but the car idles around 10 degrees and at 2500 rpm the timing is roughly 34 degrees. I'm fine being on the safe side of it. Engine is old/ slightly tired and the car is just a driver (for now). I am not looking for performance but I am scared for predetonation ruining my engine. It is a 383 2-barrel that now has a 4-barrel, intake and headers but I believe the rest to be stock.

So.. here is my problem: When I drive around 30-40 mph and let off the gas or touching the gas just slightly to maintain the speed I hear a faint sound that resembles (what I think) ignition. The sound is very constant (every cylinder seems to produce it) and I am worried that it might be predetonation happening. The engine sounds fine when I step on the gas or drive with more throttle engaged.

I am thinking that this might be caused by the vacuum advance since it only happens with out / or with slight throttle. However, reading about timing has taught me that pinging usually checked during acceleration or with a fair amount of load on the engine.

Driving around with the vacuum advance disconnected made the car run worse but it did get rid of this sound...

Could the vacuum advance be the problem and/or is it normal to hear this?

I'd love to hear what the more experienced think about this.

Best regards and thanks in advance (pun intended ;)

10 degrees before top center you will get pinging regardless if you have the vacumm advance hooked up or not. Is the green tune up specs sticker still on the inner fender? See what the timing specs are. In my 68 Dodge service manual it states the 2 barrel 383 timing is 7 1/2 degrees BTDC and the 383 4 barrel is 5 degrees before top center. I would back the timing down to 5 degrees BTDC and see if there is any pinging. If none and your happy with it , then leave it there.

 

[QOTHL]

High mileage, might have a bit of carbon build up in the cylinders/heads. Pull some spark plugs to see what they look like.

Nah, take it out on the highway and FLOOR IT!

 

[QOTHL]

Best regards and thanks in advance (pun intended ;)

How are things over there? Hearing some interesting stories about your wonderful farmers.
Second largest food producer in the world, that's amazing!

 

[Geoff 2]

Some misinformation here:
- yes, vac adv can cause pinging, but in your case, the likely cause of your pinging is stretched advance weight springs in the dist. This would increase timing in conjunction with what VA adds & maybe too much total timing.
- never heard of a factory VA unit being adjustable [ post #6 ], if still original. Some aftermarket units are adjustable.
- 10* initial timing [ post #8 ] will NOT cause pinging.
- post #4. Man Vac adv does NOT cause overheating or performance issues when correctly set up. It actually enhances performance & cools the engine.
- heat is the enemy of pinging [ detonation ]. Cold air induction & cooler running will help.
- lean mixture also causes pinging.
- the engine runs worse without VA because the engine needs the extra timing at cruise. Old, worn engines can allow oil into the combustion chambers, diluting the octane rating of the fuel & increasing the chance of pinging. Older high compression engines become a delicate balancing act with pinging.
- you could try a stronger spring in the dist, but that might also cause loss of performance.
- I doubt an adj VA unit will help. They usually have a softer spring, which will cause the VA to hang on longer. The opposite of what you need. You could try to reduce the travel of the actuator on the factory VA. Make some sort of a stop that limits travel to about 3/4 of what it is now.

 

[GMP440]

10 degrees before BTDC can cause pinging. Seen that happen in a few engines. No misinformation.

 

[4406bbl]

As long as he is not over 36-38 degrees total at 3000 rpm his initial timing is not the problem. He claims no pinging until he hooks up the vacuum advance, and performance is good. If this is the case the vacuum can needs limiting and maybe the spring tightened inside. The factory vacuum cans on mopars are adjustable if the end where the vacuum line goes is hex shape, I think for his year car it will just be a round can. It may also have a generic can on it that has way too much advance, he can read the previously linked artical on how to adjust his or a new can. It is worth the time to get it right. Back in the 70s it was common to see the vac advance unhooked so people could run regular gas and still run a lot of initial for the snappy throttle response.

 

[4406bbl]

I better clarify, 36-38 at 3000 with the vacuum advance disconnected. Nobody can give him exact advice, to get close we would need the number in inches on the vacuum can, the number on the advance weight slotted plate, and a pic of the internals below the breaker plate to see if it is already limited or has stock advance springs.

 

[Geoff 2]

All the adjustable VA units I have seen have a large hex on the end.
[1] Are these used on Chrys factory dists to provide adjustment?
[2] Are there adj VA units on Chry dists that do not have the hex end? The Hex end houses the adjustment section.

 

[Geoff 2]

Post #12.
For the 3rd time, 10* BTDC does not cause pinging. 36-38* might....

 

[4406bbl]

Yes factory distributors had the adjustable cans with hex, not sure when they started that, most of the 20 or so I have are that way. Stock initial timing on a 383 2bbl automatic was 7.5 degrees before if I recall right 0 degrees for a 4 speed. It depends what distributor he has. My 68 440 auto distributor does not have the adjustable can.

 

[moparedtn]

Another old goat speaking from personal experience here - no expertise claimed by me. Seriously.
Ok...
- Speaking stock for stock, I've always found the 383 wants a little less timing than its' 440 big brother -
in any mode. At idle, a bone stock 440 likes at least 10BTDC; the 383, a little less, but more than
5BTDC every time I had one in front of me. 7-8 seemed to work usually.
- The stocker 383 seemed to like an "all in" setting in the low 30's BTDC (with vacuum advance plugged
off, of course) while the 440 always seemed to like a little more, like 35-36BTDC.

(This is the way I typically set timing on these engines - at about 2400RPM, I go for the best direct vacuum
reading and the smoothest running, which determines the "all in" setting - and whatever that leaves the
idle timing at as a result is typically acceptable, too. The engine will tell you where it's happiest!).

(Either engine will happily motor on down the road with the vacuum advance disconnected, by the way.
You'll suffer some fuel economy and maybe even a little cooling from doing it, but it's fine otherwise).

Yes, many of the Chrysler factory distributors have adjustable vacuum advances on them (the electronic
ones especially) and a simple little hex wrench (I'm thinking 3/32" ??) is all that is needed.
Once you get your base timing figured out, then you can start experimenting with the vacuum advance -
but know that sometimes without some modifications, that rascal will never "settle down" to an acceptable
operating state - the engine will be "jittery" at cruise speeds and will "rattle" upon letting up on the throttle
to coast/slow down.
Yes, that's what you're hearing if you hear it at that point - it's pre-ignition where it's not welcome by the
engine. It most likely is caused by the vacuum advance but as others have said, it can also be caused by
an overly lean condition, too - and if the engine is running hot, well, that just makes the whole mess worse!

Today's crap gas, along with whatever modifications have been made to the engine, how old it is,
what it's sitting in, all sorts of factors lead to detonation and pre-ignition.
In my own '68 GTX with a "semi-warmed" 440, I run the Mopar electronic ignition sans vacuum advance
as a result. The car is happy (but a gas hog, admittedly) and there's no staccato music under the hood.

 

[Hemirunner]

These timing discussions are always a **** show. Lots of inference being thrown around in regards to initial vs total timing. This guy is a complete newb by his own admission. You guys might want to elaborate between initial, mechanical, vacuum and total so he understands there’s a full curve involved and every engine will be slightly different.

 

[Newkidonthebloc]

These timing discussions are always a **** show. Lots of inference being thrown around in regards to initial vs total timing. This guy is a complete newb by his own admission. You guys might want to elaborate between initial, mechanical, vacuum and total so he understands there’s a full curve involved and every engine will be slightly different.

No offense, but I do know what that all means. I hoped that my description of my problem would let you know that I know what I am talking about but am a complete newb in actually physically applying the knowledge to setting the correct timing and tuning. I have read about it for many years and know a fair bit but I never had a car to apply that on. I lack experience and therefore am not quite sure whether the sound I am hearing is normal or not.
I also have a kit means to change the curve of the distributor which is probably the next thing I will do once I figure out what causes the sound and whether it actually is detonation.