Dual vacuum advance problem

[Jimz Bird]

The dual vacuum can on the 56 does cause some confusion since it was for just one year.
Yes, the lower port on the back of the 4000 where the outer or "Secondary Spark Control" diaphragm is connected does get direct manifold vacuum. Also attached there to the brass block are the connections for the power brake booster and the dual action fuel pump.
The idea is that when manifold vacuum drops it will provide a quick retard of the timing. As the Shop Manual states; "This provides the sudden retard necessary to prevent "tip-in" detonation. As soon as the vacuum passage pressures normalize, the Primary spark control diaphragm again resumes full control of advance characteristics." See Pgs. 74-5 for more details. This was an attempt by Ford to overcome the shortcomings of the 1955 LoadO Single diaphragm distributor on a high performance V8 engine. This port is not part of the "blended" vacuum that controls the actual advance. It is a 3rd port for just manifold vacuum. It is a "goose" port.
Also the Holley 4000 Services Manuals Form C-228 & the 1956 Supplement along with the Holley Pressure Type Distributor Service Manual Form C-235 are good as they helped me to understand a little better how this all works together.

 

[scicala]

Scicala, I just checked back here and saw this post. Are you sure that fitting at the base of the carb connects to manifold vacuum? Because if it does, you will have full advance at idle and it will idle very rough, plus likely ping real bad at low speeds due to the excessive advance. And at full throttle manifold vacuum drops to zero so all your advance goes away and so does most of the power. I think you will find that this fitting has internal passages that go the venturi for the vacuum. Airflow through the venturi creates a vacuum signal for the advance.

The large brass fitting at the base of the rear of the carb is for manifold vacuum. The passage goes to below the rear throttle plates. It does not over advance because the vacuum is bled off from the vacuum line coming from the front of the carb to the dist. This line carries a combination of venturi vacuum and manifold vacuum. The link below is to a Holley manual that explains it all, and has a diagram of the system.
I believe page 10 of the manual starts the text.

Sal


http://www.tocmp.com/manuals/Carbs/Holley/H4000/C228/index.htm