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Old 01-01-2005, 11:17 PM
western star's Avatar
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Vacuum advance

How much vacuum does it take to start moving a vacuum advance. Hope this sounds right. I tested mine with a proper pump and it seemed to take a lot of air to start moving it. I thought it should start moving with a small amount of vacuum.

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Old 01-01-2005, 11:31 PM
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What does "a lot of vacuum" mean to you? Did you get any readings off a gauge? Is your vacuum canister adjustable or a factory non-adjustable unit? You should be able to move the advance by sucking on the hose with your mouth.
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Old 01-02-2005, 08:51 AM
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Look in Chilton

First, most bigger Chilton type manuals (you can usually find them at the library) will actually give specs for how much advance at how many inches of vacuum for specific models of engines.

From what I've played with a long time ago, it seems 3 to 5 inches is a common starting range.

You should be able to easily make it advance by sucking on a hose attached to the can.

I've also seen the inside mechanisms of distributors rusted up so the advance, either mechanical or esp. the vacuum, won't hardly work.
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Old 01-02-2005, 10:20 AM
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Thank you for that. Exactly the info I needed. I will check the Chiltons out.
Willys36 I was up to about 10 inches of vacuum to start moving it.
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Old 01-02-2005, 11:12 AM
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Can #

If you can find a number on the can
or on the arm coming out of it that
hooks to the distrib plate, you may
need that as cars had variations
depending on where they were sold.

Otherwise, you could hook a vac meter into a vac source, like iwth a T, then observe at what vac is starts to move and what vac it arrives at full advance.

Then, driving w/a vac gauge, you can
tell how much you can step it down
before you lose some or all of your
vac advance, helps with mileage as I found out on my 305 3/4t van.

You can also figure out your advance
curve by sticking on a timing tape and a tach and watching the advance versus rpm while somebody else slowly increases rpm (disconnect and plug the vac for this so you're only seeing mechanical advance). Then, after you
find out what you've got for mechanical (which would equal advance at wide open throttle and at any vacuum value below which the vac cannister starts to pull in more advance), you can adjust to get max advance without ping or detonation - that'll be your best power and best mileage. Lots of times you can put on a vac can with less total range of advance so you can dial in a little more initial and still not end up with too much at steady road cruise rpm when full vacuum is also in effect.
It's generally good on a basically stock engine to limit that total to what the stock specs say as it will usually be lots higher than you'd imagine unless you've messed with this topic a bunch.
Again, the specs for that should also
be in the Chilton.

You can also get vac cans where you can adjust the vac at which it starts and stop and the rate of advance versus vacuum.
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Old 01-02-2005, 12:22 PM
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The engine is a 1969 340 Dodge. I've checked the Chilton manual and it says my advance should start to move between 5 and 7.5 inches. I checked this with a pump / gauge and it doesn't start moving until close to 10. There are no leaks in the diaphram. It holds very well. Could this difference cause a slight off idle stumble in the throttle? My starts are positive off the line but from light throttle cruise speed and then with a little throttle to increase speed I will get a stumble then it catches and goes. Everything else has been checked and tweeked including a new carb. You name it and it has been changed or played with. This is why I am playing with the vacuum and timing.
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Old 01-02-2005, 01:42 PM
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Couple Possibilities

There's a number of things of course that can cause a stumble and that's why it's so much FUN!

Thinking "out loud" here -
At idle, you should be pulling full vacuum advance but no mechanical. Giving it some throttle would reduce the vac, so the vac can should back off some pretty much immediately. If the can isn't backing off quickly enuf because of a "gummed up" advance mechanism, I don't know that it would cause a stumble. Same thing from light throttle cruise - full vac, little mechanical depending on your rpms, but likely not much anyway. Again, stepping on it the vac advance should pull back some, the amount depending on how hard you're stepping it down. Delaying the pull back (losing some advance) I wouldn't think would cause it to stumble.

I woudl suspect something more in the line of accelerator pump settings on the carb not coming in to action quite quickly enuf to make up for the additional air being let in to the intake.

It's always good to check all the settings on a new carb, esp accelerator pump (or in the case of a Mopar with an AVS or AFB, the metering rods being pushed up out of the primary jets by the plunger spring to enrichen the mixture). When you step on the accelerator you open the throttle blades and the vacuum in the carb and intake goes down, reducing signal to the boosters in the carb, thereby reducing gas flow and leaning out the mixture temporarily until vacuum catches up. The metering rods in the primaries are controlled by a plunger that is pulled down by more vacuum and the spring under the plunger pushes up the plunger and the
metering rods when vacuum decreases when you step on the throttle. This pulls the rods farther up out of the jets, to a spot where the rods are thinner, allowing more gas thru. Same effect as an accelerator pump or power valve on a Holley.

I've forgotten a lot about the AFB/AVS type carbs, don't remember if there's any adjustment screw in the plunger to mechanically adjust the height, or if you have to change metering rods to ones that are thinner at the same relative position - in other words have a different amount and rate of taper, or change to a stiffer spring to push the plunger up quicker when vacuum goes down. Any one or combo of those would act like putting quicker or greater stroke on a Holley accelerator pump, or putting in a power valve with a smaller opening vacuum value.

I believe there's likely some specs on metering rod #'s, or something related, in the manual also. You'd have to get your carb # and then look the specs up for your exact carb.

Back to the vac can - if it opens late, you're not getting as much advance as you should at light cruise which will primarily reduce your gas mileage somewhat, and depending on if it
springs back more quickly because of greater vacuum required to open it, or if it's slower in springing back because of a gummed up advance mechanism in the distributor, would alter your actual rate of advance or retard some from the specs, but I wouldn't think it would be sufficient to cause the stumble. Think of it like this, you can change your timing a few degrees either way from specs and you usually won't cause a stumble like that, although it will have an effect on power and economy at any given rpm.

Make any sense?

Hope it helps somehow.

Good luck!

There's a number of things of course that can cause a stumble and that's why it's so much FUN!

Thinking "out loud" here -
At idle, you should be pulling full vacuum advance but no mechanical. Giving it some throttle would reduce the vac, so the vac can should back off some pretty much immediately. If the can isn't backing off quickly enuf because of a "gummed up" advance mechanism, I don't know that it would cause a stumble. Same thing from light throttle cruise - full vac, little mechanical depending on your rpms, but likely not much anyway. Again, stepping on it the vac advance should pull back some, the amount depending on how hard you're stepping it down. Delaying the pull back (losing some advance) I wouldn't think would cause it to stumble.

I woudl suspect something more in the line of accelerator pump settings on the carb not coming in to action quite quickly enuf to make up for the additional air being let in to the intake.

It's always good to check all the settings on a new carb, esp accelerator pump (or in the case of a Mopar with an AVS or AFB, the metering rods being pushed up out of the primary jets by the plunger spring to enrichen the mixture). When you step on the accelerator you open the throttle blades and the vacuum in the carb and intake goes down, reducing signal to the boosters in the carb, thereby reducing gas flow and leaning out the mixture temporarily until vacuum catches up. The metering rods in the primaries are controlled by a plunger that is pulled down by more vacuum and the spring under the plunger pushes up the plunger and the
metering rods when vacuum decreases when you step on the throttle. This pulls the rods farther up out of the jets, to a spot where the rods are thinner, allowing more gas thru. Same effect as an accelerator pump or power valve on a Holley.

I've forgotten a lot about the AFB/AVS type carbs, don't remember if there's any adjustment screw in the plunger to mechanically adjust the height, or if you have to change metering rods to ones that are thinner at the same relative position - in other words have a different amount and rate of taper, or change to a stiffer spring to push the plunger up quicker when vacuum goes down. Any one or combo of those would act like putting quicker or greater stroke on a Holley accelerator pump, or putting in a power valve with a smaller opening vacuum value.

I believe there's likely some specs on metering rod #'s, or something related, in the manual also. You'd have to get your carb # and then look the specs up for your exact carb.

Back to the vac can - if it opens late, you're not getting as much advance as you should at light cruise which will primarily reduce your gas mileage somewhat, and depending on if it
springs back more quickly because of greater vacuum required to open it, or if it's slower in springing back because of a gummed up advance mechanism in the distributor, would alter your actual rate of advance or retard some from the specs, but I wouldn't think it would be sufficient to cause the stumble. Think of it like this, you can change your timing a few degrees either way from specs and you usually won't cause a stumble like that, although it will have an effect on power and economy at any given rpm.

Make any sense?

Hope it helps somehow.

Good luck!

Of course, if you've gone to a longer duration cam or bigger intake, you need to change the rate of getting more gas into the manifold when you step on it . Lower number power valve for Holley, and/or different accelerator pump cam giving it a bigger and quicker shot, or in the case of an AFB/AVS, probably different metering rods at least. In either case, maybe even somewhat larger jets.

If it's an AVS, make sure the secondary air valve spring isn't set too soft and letting the air valve controlling airflow into the secondaries open too quickly. Same effect as too little gas on the metering rod or power valve/accelerator pump activation.

Oh, I'm also now remembering there are maybe different accelerator pump arm settings on those AVS/AFB carbs, like 2 or 3 little holes in the accelerator pump arm for linkage adjustment, which would also quicken the accelerator pump stroke. Likely only need to change that if you've done some fairly serious hops to the engine. But check to make sure it's in the right hole per specs even if not.
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Old 01-02-2005, 03:22 PM
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Macx...wow thank you for spending the time to post that reply. I have done all of that. The carb that was on it was an Edelbrock 600 and the new carb is a Holley 670 Street Avenger. The stumble was there on both carbs. We have done everything on the new carb including changing pump nozzles below stock and above, float levels, throttle cams have been changed back and forth, accelerator pump gauged, you name it and we've tried it. The engine has low miles on it and is primarily a sweet runner. I'm trying everything I can think of in order. This is why I am at the vacuum advance stage. I just had a friend of mine over and we put the pump on the can again. It seems to be around 12 inches of vacuum before the distributor starts to advance. I think there may be something preventing to advance linkage to move smoothly. I'll pull the dist and check it out on the bench. Maybe I'm getting close.

Last edited by western star; 01-02-2005 at 10:51 PM.
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Old 01-02-2005, 04:53 PM
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Covering All Your Bases

Man, sure sounds like you're working it one end to the other!

If you've gone thru all that, you've about eliminated everything except that distributor!

Sure does sound like there's something not right in there someplace, esp with that vac advance situation.

I've always admired the 340's = good friend of mine had one
years ago and, in spite of being pure stock cuz of his budget, it amazed lots of cars with "reputations".

Good luck with it! When you get it figured out, I'd sure like
to see what it turns out to be! I never stop trying to learn.
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Old 01-02-2005, 05:16 PM
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I certainly will.
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Old 01-02-2005, 05:48 PM
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More or less vacuum advance would not cause a stumble.Double check excessive chain play.
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Old 01-02-2005, 06:16 PM
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Re: Couple Possibilities

Quote:
Originally posted by macx

At idle, you should be pulling full vacuum advance but no mechanical. Giving it some throttle would reduce the vac, so the vac can should back off some pretty much immediately.
That would be if you were running the advance on full manifold vacuum. I would assume that, stock, it ran off of ported vacuum, as most all do.

Not to get into the "which is better" debate, I'm just saying.
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Old 01-02-2005, 10:35 PM
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Chain is fine. Engine is almost new. I tried running it from the manifold and it ran terrible. I run the vacuum above the throttle plate on the side of the carb where Holley recommends it.
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Old 11-30-2009, 08:32 PM
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Just a thoughtó are you using the carb's metered or full vacuum port?
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Old 11-30-2009, 10:28 PM
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When I had just the 750 vac sec on my rod we created the stumble by going smaller on the jets by 4 sizes. It was less with 2 sizes and gone with the stock jets. You could see it clearly with the AFR gage too.

So I would say try going 2 sizes larger main jets. I know that the main jets are not supposed to do this but solid testing showed it did.
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