As you can imagine, I have been looking for documentation one way or the other. I have found several items stating as I am that the vacuum retard is only at idle and the port provides vacuum only at idle, not off idle. Unfortunately, those are merely posts without the documentation I would like.
I did find one here on JL. From:
Ported and manifold vacuum on Stromberg carbs - E-Type - Jag-lovers Forums
And the pertinent post is by @MarekH:
"There are three main vacuum pickup points associated with the ZS carburettor and the easiest way to see them is on the final 1974 cars.
The first conventional one will be manifold vacuum - measured after the throttle butterfly and being what the cylinders actually see.
The second is the ported vacuum location as described above by David. If you look at the emissions carburettors with EGR for 1974 then you’ll see that this is where that connects and you’ll see a small drilled hole (from memory about 2mm diameter).
Lastly, the v12 used a vacuum retard capsule which operated at idle only and the pickup for that was on the bottom of the LH rear carburettor.
The Ford EDIS used by Ray and others will have an ignition map covering the whole range of vacuum from atmospheric to full vacuum. It’ll expect to see manifold vacuum, not anything ported or ameliorated by other factors.
kind regards
Marek"
His post may be referencing the V12 in particular, but I believe he is including all ZS. And the retard port is the same location as on the S2 ZS. Note Merek refers to the manifold vacuum port as a different port than the one used for ignition retard and describes it as different.
So, I am hoping Merek will have some comments. And although Merek says that it only operates at idle, and that information does agree with the only documentation that I can find and have posted above, Merek, is that indeed exactly what you meant to say, or are you just generalizing? And most importantly, do you know where you learned that information and do you have any documentation for it?
IMO, Merek is stating exactly what I am. I clearly am not trying to twist anything Merek said, only understand it and this topic. I am hoping Merek will respond.
Thanks,
Tom