Vacuum for dummies

Cougar Community Classic Cougar Maintenance Restoration and Repair
1

351W 2V in a 69. Automatic tranny with factory A/C

Hey so, after some new parts (PCV valve and hose from the carburetor riser plate to the valve, plus a new valve cover grommet for the set up) including a shiny new timing pointer (no more guessing on timing) I think I might be having vacuum issues.

So, the Distributor Vacuum Control Valve. I understand the function. I read about what it is, what it needs to do, and why it is so important. I have factory shop manuals for the car. Where I am getting confused is shouldn’t there ALWAYS be the presence of some sort of vacuum at the vacuum advance on the distributor? It seems there is not in my case. Which leads me to believe one of three options.

  1. the Distributor Vacuum Control Valve is bad, therefore not allowing carburetor vacuum to affect distributor timing
  2. the lines are incorrectly hooked up. I have the read the manual and triple double checked the connections according to the book and they are as they should be
  3. I’m not actually getting any vacuum from the carburetor through the line and thus into the vacuum advance diaphragm.

Do these valves actually fail often or are they pretty hardy? What I need is vacuum gauge to determine if I am actually having vacuum issues I suppose. But I thought I’d see your guys’ thoughts on the matter before more parts or more tools.

2

Typically, there is no vacuum at idle speeds at the distributor input. As rpm rises (typically above 1000 rpm), vacuum begins and advances the distributor.

3

Thank you… that helps a lot. I assumed there should always be some vacuum at the diaphragm. I assumed wrong. Story of my life I suppose.

4

Here is a very good article about vacuum advance.
It is a Chevy forum, but I think the principle is the same for all American cars built in this period.
https://www.chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm
I understood from there that vacuum advance at the distributor - at idle - will vary dependng if you are using full manifold Vac vs Ported Vac.
Edit: removed my car setting.

5

Very informative article on vacuum advance. Now that I more better understand the principle behind it, I can better diagnose what may be causing my issue. Thank you for that link, good sir.