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Vacuum Advance Pod
https://slantsix.org/forum/viewtopic.php?t=47138		 Page 1 of 3
Author:  duval67 [ Tue Nov 29, 2011 5:08 am ]
Post subject:  Vacuum Advance Pod
Hey guys,
I have a feeling the advance pod on my distributor isn't working whatsoever, so I'm gonna replace it. Even if that isn't the problem, I'm sure it will be one day--doubt it's ever been changed.

Where should I look to buy one? And what kind (strength? :? ) should I be getting?

The vehicle is a 1980 Dodge d150.
Author:  supton [ Tue Nov 29, 2011 5:27 am ]
Post subject: 
Should be able to test it easily enough--with engine running (and up to temp), hook up a timing light, see what timing it has; disconnect vacuum line (and plug it off), and see if the timing moves. If it doesn't move, I'd first check the routing, maybe there is an inline thermostatic vacuum valve that isn't working--although you should be able to tell just from holding your finger on the vacuum line itself.

I take it that this is converted away from Leanburn, and has a vacuum advance on the distributor in the first place. Right?

What makes you think the advance isn't working properly?
Author:  wjajr [ Tue Nov 29, 2011 7:00 am ]
Post subject: 
Remove distributor cap, pull pod’s vacuum line from carburetor, and suck on vacuum line while observing pod’s arm in distributor housing. If it moves it works. Now suck on vacuum line, block line with tongue and see if dashpot holds a vacuum, or in other words once pod’s arm is retracted, dose it stay retracted, or dose it slowly return to rest before vacuum was a applied.

If the arm stays put, the rubber diaphragm is good, if arm slowly returns to rest the pod needs to be replaced as it leaks.
Author:  duval67 [ Tue Nov 29, 2011 9:51 am ]
Post subject: 
To be honest, I don't fully understand how vacuum advance is supposed to work, what with the debate on ported vs full vacuum for the advance. But i remember my father and I concluding a while ago that is was likely a source of our problems.

I will test it when I get back from uni for Christmas and see if it is in fact working.
If it isn't though, where should I look to find a new one?
Author:  supton [ Tue Nov 29, 2011 10:44 am ]
Post subject: 
Under lean conditions (part throttle), more timing advance is desirable, as the mixture burns a bit more slower. Lighting it off sooner means more of it burns in the cylinder as opposed to out the exhaust, leading to more relative efficency (energy being used to turn the crank rather than used to heat the atmosphere).

However, as the air/fuel mixture richens up (more throttle), lighting it ealier is counter productive--since the mixture is now more rich it burns faster. It may also lead to detonation (pressures spike upward and cause the air/fuel mixture to autoignite apart from the spark plug; when the two flame fronts collide, it rattles the piston, creating the characteristic ping--and engine damage).

So, it becomes desirable to pull back some of that advance that we wanted, depending upon the mixture ratio. Generally speaking, the engine is tuned to be leanest at low loads, which has the most manifold vacuum; and richest at its highest load (or lowest vacuum). Ergo, using manifold vacuum, which drops as the throttle plate is opened up, as opposed to ported vacuum, which rises with engine rpm. [Manifold vacuum being found under the throttle plate, ported vacuum above it.]

[You'll note that this is apart from mechanical advance, which increases the advance in relation to increasing rpm. Vacuum and mechanical advance are summed together, on top of the initial advance. Mechanical advance is required as it takes something of a fixed time to get full cylinder pressure/combusting of the air/fuel mixture, while the window of time in which we want max pressure--where the piston is descending on the power stroke--is decreasing as rpm's climb. Initial advance is simply something of a baseline, a place to start from, because one can only get so much advance from either source.]

[Generally speaking, this is true for carb applications. EFI setups are similar but use computer control instead.]
Author:  duval67 [ Tue Nov 29, 2011 11:41 am ]
Post subject: 
Ahh okay, thanks!
I want to be connecting the vacuum advance to the manifold vacuum then. Gotcha.

While we're on the subject, how can I check if my mechanical advance is working properly?
Under quick, heavy acceleration, my truck stumbles, feels like it might stall, then catches and accelerates normally.
What would you consider to be the most likely cause for this? I imagine it is a combination of vacuum advance, mechanical advance and general carburetor problems (I will be checking the advance pod, cleaning the distributor and rebuilding the carb to solve the problem).
Author:  supton [ Tue Nov 29, 2011 12:10 pm ]
Post subject: 
Sounds more like the accelerator pump, which is a part of the carb. Its job is to squirt in some fuel, as the quick opening of the throttle causes a momentary lean condition--the carb has to transition from one meter circuit to another.

If you roll into it slowly, does that avoid the hesistation?

Checking mechanical advance: disconnect the vacuum line going to the vacuum advance, and plug it off. Hook up the timing light, make note of the timing at idle. Rev the engine. If no advance is noticed, then something is amiss. If the timing jumps around as you rev it, it could be sticky. If the timing jumps around at idle, though, you may have a worn timing chain.

Before tearing into the carb, I'd take a quick look to see if the throttle bushings are worn. It does not sound like it; but it's worth looking at first, before jumping into a rebuild. If they are worn, I would find a place where you can have new bushings installed, or finding a quality (not a poorly rebuilt) carb.
Author:  duval67 [ Tue Nov 29, 2011 12:30 pm ]
Post subject: 
Believe me, I've always attributed it to the accelerator pump. I really want to believe that that's the problem, but I can see a pretty distinct stream of fuel when giving it throttle.
I'll check the mechanical advance like that when I get home.

How can I check if the throttle bushings are worn?

I'm not too concerned about doing a full carb rebuild, I'm only running the stock Holley 1945 right now.

EDIT: http://www.oreillyauto.com/site/c/detai ... vi=1084384
This is the rebuild kit I was planning to buy. It doesn't contain a new accelerator pump, does it? Where might I begin to look to find one?

I appreciate all the help guys.
Author:  WagonsRcool [ Tue Nov 29, 2011 1:12 pm ]
Post subject: 
Most chryslers that I have worked on have the vacuum advance hooked to ported vac. Personally, I wouldn't run advance from (full) manifold vacuum. Here's why- you get the most advance with the highest vac. (at idle). So idle speed goes up (from previous ported vac setting) & you have to back the base idle speed adj. off until you're down to spec. All well & good (but you're not idling on base timing anymore). Now if you have any type of glitch or hiccup that affects eng operation, you lose (reduce) manifold vac, & lose advance- vac & timing go down further & you have a surge/ stall condition.
Author:  supton [ Tue Nov 29, 2011 3:59 pm ]
Post subject: 
Duval: going from the scan Dan has stashed away on his site, I think the blue round piece in the pic on O'Reilly's site is the accerator pump diaphram. So you might be all set.

WagonsRCool: interesting. Goes to show there is often more than one way to tune a setup. Although I'm not sure if that is a crutch: just what is going to make the engine stumble? Overly rich (or lean) running, worn plugs, etc?
Author:  duval67 [ Tue Nov 29, 2011 4:31 pm ]
Post subject: 
Perfect. I was thinking that, just wasn't sure.

I guess I'll have to look into this ported vs manifold vacuum a bit more!
Author:  supton [ Tue Nov 29, 2011 4:41 pm ]
Post subject: 
Well, it's easy enough to move the vacuum lines around. It might be a bit more work to play with the timing and/or fuel adjustments, but eh. It won't be hard to try both ways.

I'd just keep a log of what you've tried. You know, "pinged at 12 degrees initial on 87 octane", etc. On the last couple of cars I've owned I've kept a cash book in the car, to record fuel fillups/etc. That might be a good place to jot down tuning attempts, to keep track of what was tried and what didn't work. I like to take notes, only because when I don't figure something out after a few tries, I invariably wind up forgetting what I've tried--and wind up repeating stuff that didn't work.

Good luck!
Author:  duval67 [ Tue Nov 29, 2011 4:58 pm ]
Post subject: 
Good call, I'll do that!

I think I'm gonna work with manifold vacuum...the typical consensus seems to be that it works out better in the end. If it isn't working for me, I'll fiddle around with ported.
I'm curious as to what I've been running :S I wish I was at home so I could check all this stuff right away! Only a few more weeks though.

About the vacuum pod again though, in case it isn't working; where would I be able to find one? Just any regular auto store?
Author:  supton [ Tue Nov 29, 2011 5:45 pm ]
Post subject: 
I did a bit of google, as your O'Rielly's didn't have anything under "vacuum advance". I did find this stuff, though:

http://www.autopartswarehouse.com/sku/U ... 29332.html
http://www.autopartswarehouse.com/sku/U ... 68463.html

There was some parts for playing with mechanical advance also:
http://www.autopartswarehouse.com/sku/U ... 15754.html
http://www.autopartswarehouse.com/sku/U ... 29015.html
http://www.autopartswarehouse.com/sku/U ... 29014.html
http://www.autopartswarehouse.com/sku/U ... 2927G.html

Never bought there before, YMMV, etc.
Author:  ceej [ Tue Nov 29, 2011 5:47 pm ]
Post subject: 
Not sure where you heard there is a debate about using manifold vacuum or ported vacuum to operate the vacuum advance.

Do not connect your vacuum advance directly to manifold vacuum. It must be hooked to ported vacuum, or to a vacuum amplifier operated by ported vacuum.


2¢
CJ
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