Vacuum Advance

[Patrick]

Hey Guys,

 

I have been having an issue with my '52 B3C lately where the motor won't rev up. I rebuilt the carburetor and it is working fine. So I moved onto the distributor. The points were dirty and there was a couple original wires inside that the insulation had degraded off of. So those will be replaced. I also found that the vacuum advance diaphragm is leaking air. So I'm thinking that is the major issue here. I see NOS vacuum advance modules on Ebay but I'm wondering whether or not that's a good route to go. Do you think the diaphragm is any better in those after sitting all these years? Is there a better way to go?

 

Here's the one I'm looking at on Ebay

[Plymouthy Adams]

the vacuum advance will not prevent your truck from peak performance..only peak fuel economy at sustained speed..

[Don Coatney]

The vacuum advance has nothing to do with the engine revving up from idle speed. The vacuum advance only comes into play at road speed to adjust spark advance based on engine vacuum for the best engine efficiency.

[Patrick]

So do I need to change out the vacuum advance still? And what would else can be causing this issue? Would the little wires in the distributor shorting prevent it from reving up?

[_shel_ny]

Meanwhile back at the ranch.

 

It will cost you about $33 to find out how long a NOS vacuum advance will last.

 

I believe Terrell (someone else will need to point you. I am site link challenged) will

rebuild yours, thus no "does it fit problems" . I do not know the cost.

 

You could google Terrell

[_shel_ny]

Seller you linked confuses me. Used, new old stock vacuum chamber.

[HanksB3B]

The vacuum advance has nothing to do with the engine revving up from idle speed. The vacuum advance only comes into play at road speed to adjust spark advance based on engine vacuum for the best engine efficiency.

 

Will someone please explain the procedure for checking the vacuum advance diaphram unit. I'm pretty sure it's in the Truck Manual, but I don't have any experience and am hoping someone will share.  I have a Mighty Vac gauge that I'm itching to put to use because I never used/needed it after I power bled my brake system (I may have lost the Harbor Freight receipt so if I'm stuck with it I might as well put it to good use). I never knew what Don says in the quote.

 

Thanks,

 

Hank 

Edited May 27, 2014 by HanksB3B

[pflaming]

". . . the vacuum advance will not prevent your truck from peak performance..only peak fuel economy at sustained speed . . ." (2) " The vacuum advance has nothing to do with the engine revving up from idle speed. The vacuum advance only comes into play at road speed to adjust spark advance based on engine vacuum for the best engine efficiency."

 

The above advice, in short means, you could take the VA OFF and probably never miss it. Yesterday I too was concerned about the VA but after learning about the above, I just dusted it off and put it out of my mind. I currently have spark at the points but not at the plugs, will address that this evening.   

[Don Coatney]

Will someone please explain the procedure for checking the vacuum advance diaphram unit.

Hank 

Quick and easy way to check the vacuum advance diaphragm is to suck on it to see if it holds vacuum. The posting below may help also.

The first step in doing distributor (or any other) work is to read the book. Of interest my book did not give any tolorence levels for distributor bushing run-out. However run-out can be measured using a dial indicator and checked at speed using a variable speed electric drill and a dwell meter tachometer. A dial indicator can also be used to read contact point opening.

 

A hand vacuum generator works well to set the vacuum advance curve. I have instructed many folks on how to do this shim adjustment procedure. For a P-15 vacuum advance begins at 2 degrees @ 6 3/4" of mercury and is full in 10 degrees @ 14" of mercury. Mechanical advance for a P-15 is 3 degrees @ 400 RPM's and full advance is 9 degrees @ 1300 RPM's.Dwell angle on aa P-15 with single points is 35-38 degrees. With dual points the dwell angle will increase to 46-48 degrees.

 

Every one of these measurments and adjustments can be done at home with a few basic tools. In my opinion use of a distributor machine would be nice but unless you are building a flat out race car you can adjust everything within range without use of such machine.

[greg g]

Yes the wires in the dist can cause a problem. If your replacment wire is stiffer than the one you replaced, it will restrict the breaker plates range of movement. This will hinder both the mecanical and vacuum advance proper operation.

What vacuum source are you using for your vacuum advance circuit?

[Patrick]

I have not replaced the wires yet, but I will make sure to get the same type of wire to avoid any movement restrictions.

 

 

The vacuum line is coming from the carburetor to the vacuum advance. Is that what you mean?

[Don Coatney]

". . . the vacuum advance will not prevent your truck from peak performance..only peak fuel economy at sustained speed . . ." (2) " The vacuum advance has nothing to do with the engine revving up from idle speed. The vacuum advance only comes into play at road speed to adjust spark advance based on engine vacuum for the best engine efficiency."

 

The above advice, in short means, you could take the VA OFF and probably never miss it. Yesterday I too was concerned about the VA but after learning about the above, I just dusted it off and put it out of my mind. I currently have spark at the points but not at the plugs, will address that this evening.   

PP;

If you are quoting me do not misconstrue the meaning of my posting. I never once said it is acceptable to remove the vacuum advance as you will never miss it as that is a foolish statement and one that I would not make. That may be your thought but it is not the message I was trying to get across.

[Plymouthy Adams]

I concur with don...I just explained this to you on the telephone when you called for this exact information...I

Edited June 21, 2014 by Plymouthy Adams

[wallytoo]

PP;

If you are quoting me do not misconstrue the meaning of my posting. I never once said it is acceptable to remove the vacuum advance as you will never miss it as that is a foolish statement and one that I would not make. That may be your thought but it is not the message I was trying to get across.

the larger trucks don't have vacuum advance, only mechanical. i don't miss it.

to the OP, you may have a fuel delivery issue or carb issue, although arcing wires in the dizzy won't help. how do you know the carb is working fine?

[pflaming]

 I was amiss to put out the quotations and I humbly stand corrected.  

Edited May 28, 2014 by pflaming

[HanksB3B]

I think my "Distributor Woes be Gone " threadhttp://p15-d24.com/topic/35791-distributor-woes-be-gone/ be helpful as there were many forum members that made excellent observations and offered excellent advice. A snapshot of your distributor would help. 

 

Hank 

Edited May 28, 2014 by HanksB3B

[greg g]

So here is my theory which I toss out for those with a better explanation to poke holes in. Back in the day ignition timing was set manualy with a lever on the sreering wheel. It was likely a set and forget deal unless the driver encountered conditions where more advance was needed like climbing a hill or when less abvance was needed like descending a long hill. Low compression rendered fine adjustments unneeded. When things got more sophisticated some engineer came up with centrifugal advance to adjust timing based upon rpms. This worked fine. Then some brighter engineer came up with adding vacuum advance toadjust timing according to rpm and load as determined bt throttle position, as rpms can decrease under load, but avacuum signal generated by air flow through the carb. So vacuum advance anticipates and enhances centrigugal advance for more effective managment of spark timing decided by actual engin demands and loads. This control is more necessary as speeds loads and compression increases. Hencee a briggs and stratton can do just fine with static timing while amodern vehical needs a bit more intervelntion to run well for all demands.

[Plymouthy Adams]

timing on the lever was needed for starts...to prevent your arm from breaking to a lesser degree over an advanced setting that allows for kick back..I remember well retarding the timing to start even with electric starter then tweaking it up for a faster smoother idle..centrifugal advance more than made up for this ailment..the introduction of the vacuum advance was to take advantage of leaning out a engine at speed with lighter load enhancing fuel economy

[pflaming]

May have also been a 'rolling hills' compensater to reduce down shifting.

[TodFitch]

So here is my theory which I toss out for those with a better explanation to poke holes in. Back in the day ignition timing was set manualy with a lever on the sreering wheel. It was likely a set and forget deal unless the driver encountered conditions where more advance was needed like climbing a hill or when less abvance was needed like descending a long hill. Low compression rendered fine adjustments unneeded. When things got more sophisticated some engineer came up with centrifugal advance to adjust timing based upon rpms. This worked fine. Then some brighter engineer came up with adding vacuum advance toadjust timing according to rpm and load as determined bt throttle position, as rpms can decrease under load, but avacuum signal generated by air flow through the carb. So vacuum advance anticipates and enhances centrigugal advance for more effective managment of spark timing decided by actual engin demands and loads. This control is more necessary as speeds loads and compression increases. Hencee a briggs and stratton can do just fine with static timing while amodern vehical needs a bit more intervelntion to run well for all demands.

 

Plymouth history which may not be exactly the same as for other makes and models:

 

In 1931 Plymouth introduced a freewheeling transmission to make shifting easier. As part of that introduction they found they needed to add some sort of automatic spark advance control, so they added a vacuum advance. Period literature indicates that they had issues with the engine stalling without the automatic advance. My guess is that the typical driver really did do a "set and forget" on the spark advance lever and they often did not set it properly. If the engine was directly coupled to the drivetrain then the momentum of the car made up for a number of sins and kept the engine turning over. With freewheeling those driver skill issues became more evident.

 

For 1932 through 1934 they decided that they could just get by with a centrifugal advance.

 

In 1935 they when to using both a centrifugal and vacuum advance. I believe the called it "perfected ignition" in the sales patter of the day.

 

As I understand it, optimal spark advance in terms of crankshaft rotation depends on RPM. I think that basically the flame front takes a certain amount of time to propagate through the cylinder and the higher the RPM the earlier in the rotation you need to ignite the mixture to allow for the same propagation time.

 

And also, as I understand it, the flame front propagation depends on the density of the charge of air/fuel mixture. And the density of the charge can be estimated by the manifold vacuum (lower vacuum means more fuel/air being pulled into the cylinder).

 

So you want to have both your advance mechanisms working properly to get ignition at the correct timing.

 

I haven't read the service manuals of new cars, but as recently as the '91 Jeep that I once had the principle is the same but implemented differently. Instead of having a vacuum advance and a mechanical advance in the distributor they sensed the RPM off the flywheel and the "manifold absolute pressure" with another sensor and then did a table lookup in the engine control unit. (By the way, many of the ECUs of that era were based on Motorola microprocessors which had a nifty interpolated table lookup feature built into them just to handle this type of thing.)

[jeffsunzeri]

A few comments:

- If you have a damaged vacuum advance diaphragm, you will have a vacuum leak which will cause poor idling and other issues.

- The purpose of the vacuum advance is to compensate for fast throttle opening movement from idle. Beyond idle, it has no function.

- A failed vacuum advance will exhibit a lag when the throttle is snapped open from idle.

- Industrial engines and other engines which are intended to run at steady throttle openings, without rapid opening from idle, don't need and are not equipped with a vacuum advance unit.

 

If an NOS vacuum advance unit has been in a box for a couple of decades, it may well be good. My experience is that 40-70 year old units operate just fine.

The advance unit is easy to test with lungs providing sufficient vacuum for most to operate.

A vacuum leak caused by a broken vacuum unit won't cause an engine to lack power or high RPM operation.

A bad advance unit (centrifigal) will cause an engine to lack power and not achieve high RPM operating speeds.

Incorrect timing and fuel blockage will cause an engine to not rev up.

Edited May 28, 2014 by jeffsunzeri

[Plymouthy Adams]

A few comments:

- If you have a damaged vacuum advance diaphragm, you will have a vacuum leak which will cause poor idling and other issues.

- The purpose of the vacuum advance is to compensate for fast throttle opening movement from idle. Beyond idle, it has no function.

- A failed vacuum advance will exhibit a lag when the throttle is snapped open from idle.

- Industrial engines and other engines which are intended to run at steady throttle openings, without rapid opening from idle, don't need and are not equipped with a vacuum advance unit.

 

If an NOS vacuum advance unit has been in a box for a couple of decades, it may well be good. My experience is that 40-70 year old units operate just fine.

The advance unit is easy to test with lungs providing sufficient vacuum for most to operate.

A vacuum leak caused by a broken vacuum unit won't cause an engine to lack power or high RPM operation.

A bad advance unit (centrifigal) will cause an engine to lack power and not achieve high RPM operating speeds.

Incorrect timing and fuel blockage will cause an engine to not rev up.

Edited by jeffsunzeri, Today, 06:04 PM.

 

 

a fault in your theory above..the action of the centrifugal advance is the compensation from idle to higher rpm....in the case of WOT (fast) throttle it is the mechanical advance that does the job and the very single reason that the vacuum advance will not hinder the performance aspect but relates directly to the sustained throttle condition and lesser load that enables the engine to burn leaner due to the additional advance in timing by the vacuum advance unit...vacuum disappears almost completely in WOT...and only increases as the engine builds to the speed matched to the throttle opening under load..this is a longer response time over that of static engine running less load..

 

NOW the mechanical advance fly weights  needs to be free to pivot and the spring of the correct rate and attached in position...your built in mechanical limit needs also be proper..vacuum advance takes the total timing, which is added to static + centrifugal an additional (approx. 11 degrees) for the max timing advance.

Edited May 28, 2014 by Plymouthy Adams

[Patrick]

Okay, so the vacuum advance obviously isn't causing my problem at this point. I have rebuilt my carb twice and my stepdad worked through it as well to double check my work. He's been doing this since well before I was born so I assume if he says it's good then it probably is. I have the distributor out of the engine right now and I have disassembled and cleaned it and lubed it. I am waiting to completely reassemble it until my new vacuum advance gets here but then I will reinstall it and try again.

 

Let's assume for a second that my carb is just fine. is there anything else that could cause the engine to indle but the moment you give it any throttle it bogs down and dies? It's also hard to get it started, it takes quite a bit of cranking before it starts.

[Plymouthy Adams]

sometimes the very points themselves will short out internally in the distributor..is you contact wire inside shielded and routed against shorting to the housing...were the flyweights inside the distributor free to pivot with spring attached?  further have you checked to see if your shooter is operational as it is this burst of fuel that allow transition from the idle circuit to the run circuits within the carb..and are these internal passage truly open without restrictions...what is your base line timing?  

 

is this bogging down while running at idle or does it seem to take the gas well when out of gear and shop testing...if so and failing under load..check your coil to distributor high tnesion wire and also the condenser..condenser will break down under load abut run fine static...usually a burnt bluish tint on the contact points signify a bad condenser..but not always....

[Jerry Roberts]

.... is there anything else that could cause the engine to idle but the moment you give it any throttle it bogs down and dies? ....

 There is a wire in the distributor that often causes this problem . This is a finely braded wire that must move every time you use the throttle . If it is broken on the inside or grounding out where it shouldn't , then you will have this problem . I think that  Plymouthy referred to this wire .