Vacuum port S2, 1970

My 1970 S2 OTS has two vacuum ports. One is on the bottom of the front carb, painted pink. (1st picture).
The second port is on the manifold, rear passenger side of engine, between the fitting for the vacuum canister and the water tube going to the firewall (also painted pink). What is the difference relative to use?

The pipe on the rear of the manifold is ‘manifold’ vacuum and is attached to the vacuum switch on the back of the air filter housing and then another hose goes to the ‘vacuum motor’ that moves the air door on the front of the air filter housing. The ‘vacuum switch’ is a thermo regulated switch that is open to allow warm engine air (exhaust heated) to enter the air intake till the engine reaches operating temp.
The port on the bottom of the front carb (first picture) is actually manifold vacuum also, or at least I think so. This is discussed more completely in the following.

This was 12/16.
I have not yet drilled my carb as some others have done but I am considering it.
Others may have a different view.

Regards, Joel.

Joel, Thanks very much for your reply. I had not even considered the operation of the air filter. I see from your post that you are considering drilling your intake system for vacuum advance or retard. The 1970 and up Jaguars had vacuum retard for emission control. Retarding at idle increases head temperature which is completely gone at throttle up. If you are thinking about advance for your engine, be very careful as you can end up with two much advance. A total of 45 degrees may be OK for 6 cyl JAGUAR ENGINE… I have gone as high as 55 degrees on V-8’s. There is a great article by Dick Maury that can be accessed via google by typing (vacuum retard on engine) or ( vacuum advance versus vacuum retard - Georgiajag

Joel, I checked the vacuum port on the front carb and it is open and operational.

I think the vacuum port on the front carb,coming off the bottom flange, is manifold vacuum and not ported vacuum. David Langley explained how he would and did create a ported vacuum source on the Z-S carb. As I understand what he did, he drilled a small hole at an angle so the opening was ‘in front of’ the carb throttle plate (to the air cleaner side or the out side of the throttle plate). This way the vacuum is not available to the distributor until the throttle starts to open. This is my understanding and if it is incorrect I would appreciate someone correcting me and providing the correct explanation.
Now to correct something I had written earlier.
The thermal switch on the air cleaner is C.33025 ‘Thermal Senser Unit’. It is installed on the back of the rectangular air cleaner. A hose from the rear of the manifold to the thermal senser unit, then a hose from the thermal senser unit to the ‘vacuum motor’ on the front outside surface of the air cleaner ‘snout’ (my term). When first starting a ‘cold’ engine the thermal senser unit is open and the vacuum closes the door on the front of the snout until the engine warms and warm air from the exhaust manifold causes the thermal senser unit to close and then the door in the snout opens and ‘fresh cool’ air enters the air cleaner.
Hope this helps and does not confuse.

Regards, Joel.

Joel, thanks for the update on the air cleaner/hose connections. I can now install them properly.

On your comments of ported vs non ported vacuum. Ported vacuum simply delays the vacuum advance at idle. Non ported give full vacuum from the distributor, assuming both vacuum ports are the same size. In either case, all vacuum advance is lost under FULL throttle. I like non ported vacuum advance since when motoring under light throttle you have more advance and better fuel economy. Too much advance can cause hotter engine operation. It is very easy to check idle and cruise advance with a timing light and varying the throttle position. When using ported vacuum at idle, you are back to static timing of 10 degrees.

The N.A. Series 2 were fitted with three versions of Strombergs (serially). The first two versions had no vacuum port as the distributor was not fitted with a vacuum servo. The distributor supplied only centrifugal advance. Each of the three used a different metering needle.

The first type of Strombergs had a bypass (“decel”) valve on the front carb only and it was vented to the manifold through a short pipe. The back carb had the bypass valve area blanked off.

The second type had bypass valves on both carbs.

The latest type was the EX/EM carbs and had a bypass valve on each carb and a vacuum port out the bottom of the front carb. This was for the distributor vacuum retard.

The way to identify them is the number stamped on the bottom of the mounting flange. The first type is 3165F and 3165R (front and rear), the second type is 3207F and 3207R , and the third is 3305F and 3303R.

Cars fited with the first and second set ups used coil springs for return on the secondary manifold throttle plates whereas the third had linear springs attached to “Y” brackets beneath the carbs.

Bit of a thread hijack- The Stromberg bodies on my early Series 1.5 are labeled 3165SF and 3165SR. Any idea what the “S” represents?

No idea what the S means. The 3305’s I have show a 1 behind the F and R. (One pint for lunch that day?) The information I posted comes from the literature which is only the 4 numbers and the F or R.

Correction – I type faster than I think. The “third type” are 3305F and 3305R.

I’m trying to post pictures which show the port drilling in the throat of a new 3305F.The drilling appears to be on the very edge of the butterfly so I would think it would be more port vacuum than manifold vacuum.

How do you post pictures?

Have a look in #user-guides-and-how-tos.

Here is the guide about posting photos:

here is a picture of one of my 69 Strombergs, there is a ghost S in front of the numbers on both.

this threw me for a while, but I never gave it much thought after learning what they should be.
If you look close it almost looks like a double stamped S too. I wrote it off as someone leaning on the machine or something
Maybe the same guy was working the stamping machine for Doug’s and leaned on it after the numbers!.. burp… hiccup… oops…


here is the rear carb, no S, but clearly double stamped.

Notice drilling is barely showing

The stamping on mine is pretty clear- perhaps before the 3 pint lunch?

Hi, Joel

Yes old post, but timely nevertheless.

I have a hose coming from the rear of the manifold that’s plugged and hanging. There is a hose “looped” between the two inlets to the thermal switch on the back of the rectangular air cleaner. I have no hose connected to the ‘snout’ of the air cleaner.

From your post, the hose from the rear of the manifold should go to one of the inlets of the thermal switch, and from there to the inlet at the front of the air cleaner. If that is correct I’ll give that a shot.

FWIW- The engine starts right up, and runs great with a lot of power and pep, but the idle is a bit rough. Could the way the PO connected the hoses have anything to do with that? Also, can you think of a reason why the PO would do that with the hoses?

Thank you

Your description “From your post…” describes the correct routing as I understand it. If the plugging of the vacuum line at the rear of the manifold is complete (no leaking) I do not think it has anything to do with a rough idle.
The previous owner may have plugged the vacuum line if he thought the car would not benefit from warmed air in the intake. If the vacuum motor on the snout is not working he may have decided to disconnect it. If that is the case the thermal switch doesn’t need to have the ‘looped hose’ as it serves no purpose.
As an aside my vacuum motor doesn’t work and I have been unable to source a replacement. Good luck with your car.

Regards, Joel.

As Joel has indicated it looks like the previous owner of your car may have disconnected some of the “emissions mitigation magic” that Jaguar added to the 1970s cars, possibly because the vacuum motor or other component stopped working. Your car would also originally have been fitted with a vacuum retard (not advance) distributor capsule which was designed to reduce emissions at idle. It would not surprise me a lot if that retarded ignition at idle resulted in some roughness as a byproduct.


Do you see a down side that I reconnected the hoses to the air filter box?

And how would I know if the vacuum motor and other components may have stopped working?

If the retarded ignition at idle is causing the roughness at idle, what’s the remedy- ?
So many questions from your single post. Thank you.


Carlo - I wouldn’t encourage you to circumvent the emission controls… But if a guy wanted to have his Jag run more like a Series 1, he might want to richen the fuel with an adjustable needle, take out the downstream butterflies and get a distributor that emulates an S-1 curve. For off road use of course. Some experimentation may be necessary. Get it running correctly stock first though.