[quote=“davidxk, post:20, topic:432462”]
As I understand it, the port used for the vacuum retard module monitors manifold vacuum,
[/quote],
I would have thought that, but that is not what the bulletin says, it says it picks up manifold depression when the throttle is closed. It does not say it reads manifold depression all the time. If indeed all they wanted was manifold vacuum all the time, why would they have not simply picked it up off the back of the manifold at the same spot they picked up vacuum for the other emission devices? Instead, they must have worked with ZS to drill a special port at the proper spot in the carb to accomplish exactly what the wanted, vacuum at idle only. And we probably agree there is no advantage to retarding timing at speed so this makes sense.
The chart, IMO, does not clearly say the capsule continues to retard ignition, it clearly says these are the specs for checking the capsule for proper operation. That that does not say the vacuum retard is 100% on or off, but I do read the bulletin as saying it is only used at idle.
David, at this point, unless we can get some more specifics I am not sure we can come to a definite conclusion, at least I cannot. I am not insisting that my reading of the bulletin is correct. IMO, it all hinges on what vacuum is actually read at the new carb port. Is it simply manifold vacuum or is it, as the bulletin says, manifold vacuum with the throttle closed being reduced, and granted it does not say by how much, when the throttle is opened. And is there any significant vacuum at that port when the car is driving?
In my view, I would like to put a gauge on that port and see what is there. But neither of our cars are so equipped to my knowledge. Anyone with a 1970 wish to check and report back?
Tom
The sentence following the pne you quote says “Opening the throttle reduces the vacuum and allows the distributor to advance by spring action”. It does not say that opening the throttle cuts off the vacuum source. I guess that we are both reading the same text and interpretting it differently. I agree that a measurement of the vacuum from the port could help.
Yes we do agree on that!
Any takers on providing us some readings? I would be interested in knowing what really is going on here. Just how much vacuum is at this port at cruising and in particular how much retard actually does occur while off idle.
Of course even at that we may not all agree. If the capsule provides 10 degrees total retard, and at cruise there is still 2 degrees, is that significant? Again, I can see it not being a 100% cutoff, but I would still find it interesting to know 
Tom
I love it when you folks produce these old documents,I photo them and store them in a email file along with other trouble shooting tips and tricks that are offered up
Cheers
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
On another thread, Paul has taken some vacuum readings from the carb port that is used for vacuum retard. This is what he reported:
Tom,
I hooked up a pressure gauge and Rev counter under the bonnet and recorded the results. The drop after idle was dramatic but then tailed off to around 1500.
Hope this helps!
600 - 18inHG vac
750 - 22
1000 - 8
1200 - 2
1500 - 1
1750 - 0
When you compare his readings with those in the Jaguar bulletin I posted above, one can see that with 5 in mg, the retard module provides 0-2.5 degrees movement. It does not specify less that 5 in mg, and since 5 is 0 to 2.5, I would assume less than 5 in mg is about zero retard.
Since by 1200 rpms, Paul only had 2 in mg,and no vacuum above that, apparently there is not retard above 1200 rpms.
So the conclusion based on Paul’s readings seem to agree, IMO, with the Jaguar bulletin- vacuum retard is only at idle. Thus it will not affect the engine performance off idle, or at least above 1200 rpms. So again there seems to be no performance gain from disabling a vacuum retard unless one drives with rpms under 1100 to 1200. yes, there will be some increased heat which is what the timing retarding does, but with the twin fans of the S2, I do not see that as an issue with an S2. Less pollution, no performance loss seems like a good trade to me.
Tom
Everyone,
Great discussion, and a lead on the replacement vacuum capsule. I’ll look into getting everything back to original condition.
Dave
Thanks for the help!
Dave
How about keeping the vac retard capsule but fitting a mechanical ignition advance to give an extra , say , 8 * at cruising speed ( 10 % better fuel economy ) - get the best of both worlds 
Here’s the video showing the results.
Mark,
It goes without saying that the distributors provide mechanical advance independent of the vacuum capsule fitted. Depending on the distributor model fitted, the mechanical advance gives a maximum of between 22 and 40 crankshaft degrees of advance (11 to 20 distributor degrees) based on engine rpm. The vacuum capsule (if the distributor has one) provides additional advance, or counter-acting retard based on an engine vacuum source. The Series 1 cars (40617 model distributor, or 41060) have a vacuum advance capsule which can provide and additional advance of up to 16 crankshaft degrees. The Series 1.5 and early Series 2 cars (41207 model) have no vacuum capsule, so only mechanical advance. The later Series 2 cars (41322, 41323 and … model distributor) have a vacuum retard capsule that reduces advance by up to 10 crankshaft degrees. The timing control of these distributors is, of course, extremely crude compared to modern electronic alternatives but, as you said, is an attempt to provide the “best of both worlds”. For maximum power and efficiency the engine requires more advance as revs increase. When lightly loaded, even more advance is beneficial to reduce fuel consumption - hence vacuum advance capsules). When idling in traffic, a hotter engine produces less NOX emissions - hence vaccum retard capsules. It’s all a compromise of competing requirements…
Use the series 1 curve and vacuum advance if you want the car to run well.
If you want to reduce smog, buy a Prius.