Chuck:

I also had the 3-way and 2-way valves on my 71. However, the dashpot on the rear carb was routed to the vacuum pipe on the rear carb. The vacuum source for the 2/3 way valves was routed from the vacuum pipe on the front carb.

As psfred said, the dashpot was used to keep the throttle plates slightly open on decelleration.

My system was "supposed" to work (it never did) by utilizing the 2/3 way valves and relays and switches to advance the timing whenever either the A/C was turned on or the 180C switch on the manifold closed. This then increased the idle speed. Also, the vacuum advance was, I believe, inhibited until the 65C (I think it is 65C) switch closed as was total current to the REAR choke.

I had a diagram of my setup I made once and I had a vacuum diagram I copied from an old engine manual but since I scrapped the whole setup and installed Webers I don't have them anymore.

The vacuum source pipes on my Zeniths were below the throttle plates so that really produced more manifold vacuum rather than "s-port" which would be above the throttle plates. This meant that there was a high vacuum at idle/low speeds and none (or close to none) at high speeds when none of the vacuum stuff was needed anyhow. I think this might be part of the problem when people disconnect the stuff and run a vacuum line from either carb to the advance side of the distributor. You are pulling full vacuum advance at idle and it drops off as you increase speed. Opposite of "s-port" vacuum.

Not sure this helped you. I have a box of old valves and relays if you ever want to add to your "collection"!!!

Dan

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Dan Taylor/ Tulsa, OK MBCA
'84 300D/'90 Jaguar XJ6/XJ40