Vacuum advance problems

[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:

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.

[ward 57]

Sounds more like fuel flow issues not spark advance. Failing fuel pump or clogged filters at the carb or even in the tank.

[swatson999]

Fuel flow has been checked. Filter is good (in fact, at the moment, no filter at all until I get a replacement that fits). It was the first thing we checked.

And it'll rev up just fine in the garage.

Let me repeat...with the ported line disconnected, 6 degrees. With the car running, "plug in" the ported line and it instantly jumps to 34 degrees. That doesn't seem like a fuel flow problem to me.

With the ported line disconnected and that fitting plugged on the diaphragm, timing at 10 degrees, she runs reasonably well on the road at moderate speeds, as expected.

The manual indicates that at idle, there should be NO vacuum on the ported line, and yet we seem to have full vacuum there. I don't understand it.

[T-Bird Bob]

Hello Watson,
The reason is that your throttle is not fully closed at idle. So maybe you are idling high or have other problems.

Remember, the vacuum is drawn from a port directly close to the throttle. It is placed to be above the throttle when at idle (so it is at ambient pressure) and gets below the throttle immediately when the throttle is opened ever so slightly.

See, your pre-1957 ignition has vacuum advance only. And the carb uses a combination of manifold vacuum and venturi vacuum to adjust ignition timing. For cruising it uses manifold vacuum. When driving fast with some throttle opening, it will use the venturi vacuum which is a constant function of air velocity. The spark valve switches to whichever value is higher (higher vacuum means lower pressure of course).

So when your throttle is slightly open, it opens the manifold vacuum port and the ignition "sees" full manifold vacuum.

So you got some fuel mixture problems that make you open the throttle to maintain idle RPM in Drive (Ask me how I know!). And your acceleration pump is clogged.
There is no checkball in the vacuum passages for the ignition on automatic cars, but for manual cars there is (I have no idea why). But there is a check ball in the fuel passages to the accelerator pump.
Since your talk about 6° I assume you have an automatic car.

In general, the carb is not hard to rebuild. but a lot of people still get it wrong.
There is a nice video from "Mikes carburetors" on youtube, you can do it yourself.

Here is a picture of the passages:

Good luck.

Bob

[Alan H. Tast]

Silly question: how many of your read my article on distributors and vacuum advance mechanisms in the April-May 2023 issue of Thunderbird Scoop magazine? You know, the magazine VTCI publishes for its members?