1956 dual-chamber vacuum installation

[swatson999]

I have a 56 with Holley 4000 carb and dual-chamber vacuum advance. We're having trouble getting this set-up to work properly. The symptoms:
Following the instructions, disconnect the ported vacuum and set initial advance to 6 degrees. Good vacuum from the manifold. Connecting the ported line, from the carb, the advance instantly goes to 34 degrees. Here's what's been done to try to understand and solve this:

Checked fuel flow, filter, etc. All good.

Check all vacuum lines - no leaks that we can find, good vacuum (18" or so) from the bottom of the carb at the throttle plate.
Replaced dual diaphragm with a rebuilt unit - no change.
Replaced spark control valve - no change.
Removed carb and looked in the chamber at the bottom where the port to valve is - no check ball was present, but all passages are clear; used carb cleaner throughout to ensure good clean passageways.
It's as if the ported vacuum line is *always* pulling full vacuum, but from the manual, this line should be closed at idle, opening under acceleration, and the control valve retarding the spark slightly under high acceleration and load to prevent pinging.
What the ever-loving heck is going on here? Should there be a check ball in that chamber? Or not? Why are we getting such massive advance from the word go? The carb was rebuilt about a year and a half ago, and has seemed to work just fine until recently (although I did have occasional instances of the engine not making power during high acceleration events, maybe 1 ever 2-3 months, until recently, when it just flat bogged down and stalled under any kind of load and acceleration, consistently). Something has changed or broken, but we are stuck...any advice is greatly appreciated.

 

[Hernando Muñoz]

Try closing the high side vaccum with a plug and only use the low side vaccum port at your Holley carburetor.
You will have good vaccum at idle but it will be reduced as soon as you open the throttle butterfly port at the lower side.
And then do the opposite: the high side vaccum plugging the low side to only reduce advance under high acceleration.

 

[74 Harley]

Ported vacuum should not have any vacuum at idle. The problem is in your carburetor. I haven't had a Holley 4000, I wish I could help more.

 

[dmsfrr]

Ported vacuum should not have any vacuum at idle. The problem is in your carburetor. I haven't had a Holley 4000, I wish I could help more.

swatson999, here's a (hopefully not too small) simplified diagram. The ported vacuum should be effectively zero with the throttle plate closed at idle.

 

[Bruce Hardy]

You should have vacuum at idle through the close throttle . As soo as you crack the throttle, the other side of the dual advance should get vacuum to advance though acceleration. PS Those dual vacuum ports were nothing but a headache. Trash it and install a mechanical advance distributor with one advance . you'll be happy you did.

 

[cokefirst]

It sounds like you have the vacuum lines reversed.

 

[swatson999]

It sounds like you have the vacuum lines reversed.

No, they're correct.

 

[dmsfrr]

Sharing this image just because I have it...

 

[swatson999]

OK, guys, none of this is helping...I have all of the manuals (Ford Shop Manual, Holley Carb, Holley Distributor, etc.), and the diagrams, and the descriptions, etc. Telling me that I should change the distributor, or that you have no experience with a 4000 carb, or that the ported vacuum should be low (and other things I stated in my first post) isn't really adding information to the discussion.

I appreciate the inputs, but I'm hoping *someone* has some suggestions that we haven't already tried, or has some experience with this setup and can offer some ideas.

 

[cokefirst]

do you have a vacuum gauge? The line going to the outside diaphragm of the distributor should have full vacuum at idle and the inner diaphragm should have near zero vacuum at idle. Once you go off idle. the vacuum to the outer diaphragm will reduce and the vacuum to the inner diaphragm will increase. I have both experience with the distributors and the carburetors, but sometimes it is difficult to diagnose a problem without hands and eyes seeing what is happening. If those vacuum indications are not as stated, you have an internal problem with the carburetor.

 

[Pink T-Bird]

Is it also possible you have the wrong spark valve (bypass vs non bypass style) so it is not behaving as @cokefirst describes? Your vacuum gauge test should tell you, or I guess you could pull it out to look for the bypass passage if you have not already.

 

[swatson999]

Replaced the existing one with one that has the correct part number from the supplier. I assume the old one was correct...same size, shape, appearance, holes, etc.

 

[swatson999]

the inner diaphragm should have near zero vacuum at idle. Once you go off idle. the vacuum to the outer diaphragm will reduce and the vacuum to the inner diaphragm will increase.

Yes, we know this. As stated, the vacuum at the inner diaphragm is WAY not zero at idle.

I don't have, but can get, some vacuum gauges. In the meantime, our next step is re-check all the *other* vacuum lines out of the brass block. Perhaps we have a slight leak somewhere else that is screwing up the balance between two ports.

 

[cokefirst]

If you have vacuum out of the connection to the front of the carburetor (that is the connection that goes to the inner vacuum chamber of the vacuum advance) then there is something wrong with the carburetor. At idle there should be no vacuum at that port.

 

[swatson999]

If you have vacuum out of the connection to the front of the carburetor (that is the connection that goes to the inner vacuum chamber of the vacuum advance) then there is something wrong with the carburetor. At idle there should be no vacuum at that port.

Again, "something wrong" isn't much help. We *clearly* have vacuum at idle from the port. The question is WHY?

We pulled the carb, checked and cleaned all the passageways we could, verified the lower port into the throat is clear, same with the upper to the venturi. We can see the spark valve moving at various times. And yet, at idle, there's enough vacuum to overcome the manifold vacuum on the advance and take it from 6 degrees advance to 38 degrees advance *at idle*.

There's one thing that's confusing us: the Shop Manual says that off or at idle, the spark valve is closed because the manifold port to the valve is *above* the closed throttle plate. Ours doesn't appear to be, but rather is *below* the fully closed throttle plate. We don't understand this discrepancy. I can post a pic if needed.

 

[Bruce Hardy]

Will the throttle plate go below the port if you adjust your idle screw? Or is there another vacuum port that will open below the throttle plate that you can use instead of the one your suppose to be using.

 

[swatson999]

No. We tried that (adjusting idle screw). The plates are fully closed.

I don't know what you mean by the second question.

 

[Bruce Hardy]

Is there a second port somewhere that in the primary that is below the throttle plate that will let you use that port instead of the one that doesn't work.

 

[swatson999]

Is there a second port somewhere that in the primary that is below the throttle plate that will let you use that port instead of the one that doesn't work.

This makes no sense. Firstly, no, there is no "second port" that we can use "instead of the one that doesn't work". Secondly, the problem that we see is:

the Shop Manual says that off or at idle, the spark valve is closed because the manifold port to the valve is *above* the closed throttle plate. Ours doesn't appear to be, but rather is *below* the fully closed throttle plate.

IOW, it's ALREADY below the plate, so why would a second one, even if it existed, below the plate make any sense?

 

[dmsfrr]

In your first post you mentioned the possibility of a missing check ball... maybe the carb needs a closer examination by someone who's very familiar with them?

What are the chances it's not the correct carb for a '56 T-Bird? IIRC they are a one-year-only application. Who knows what's happened in the 67 years since new...
Can you attach a photo of the ID & List number on the bottom rear flange of the carb? And the whole carburetor... Example photo.