Mystery vacuum port

Talk about the E-Type Series 3
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jagwit
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#1 Mystery vacuum port

Post by jagwit » Mon Jun 12, 2017 3:51 pm

I found a vacuum port on my car that must have been open for years....

It sits on the RH bank, on the front banana branch, between the front and rear banana intake branch, below the larger vacuum tube that connects the front and rear banana intake branches.

What is it for?

One can see it here on this pic of spare RH manifolds. Its the small one below the larger ports that link the front and rear manifolds:
IMG-20170612-WA0008.jpg
IMG-20170612-WA0008.jpg (149.2 KiB) Viewed 3055 times
Here it is on the car, taken but it might be hard to actually spot it:
IMG-20170612-WA0009.jpg
IMG-20170612-WA0009.jpg (108.25 KiB) Viewed 3055 times
Best Regards
Philip
Jag: 72 E-type V12, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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71 V12
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#2 Re: Mystery vacuum port

Post by 71 V12 » Mon Jun 12, 2017 6:16 pm

They connect to the air filter intake, air flow control flap. I believe the flaps were designed to assist very cold starting. The seals in vacuum units in mine leaked so I blanked the vacuum ports on the manifolds on both sides.

There may be one on the left bank to the dizzy - not sure off hand.

I blanked all mine off with silicon caps available on eBay. As I mentioned in anouther post there is multitude of potential induction leaks on a V12, ensuring there are no leaks is essential to obtain correct air/ fuel mixture.

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42south
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#3 Re: Mystery vacuum port

Post by 42south » Mon Jun 12, 2017 11:06 pm

Hi guys
Yes that's correct, that port had a vacuum line to the flap in the forward portion of the intake trumpet on the carburetted cars. I also have disconnected mine, and just blanked that port. There is one on the other side as well.
Its not for the distributor, that vacuum comes from the LH rear carb, either on top if you use a vacuum advance module or underneath if the car has a vacuum retard module fitted, the retard module was only fitted to the 1971 year I think.
Mark Brown
1971 S3 Etype
Son: when I grow up, I want to be a pilot
Dad: You’ll have to choose.

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John_Mann
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#4 Re: Mystery vacuum port

Post by John_Mann » Tue Jun 13, 2017 10:52 pm

42south wrote:Hi guys
, either on top if you use a vacuum advance module or underneath if the car has a vacuum retard module fitted, the retard module was only fitted to the 1971 year I think.
Hmm, I'm learning something new every day! What is this advance or retard module you speak of, Mark and how can you tell the difference? Also, irrespective of the position of the vacuum line for the distributor, I'd have thought that the vacuum would be the same?

Cheers;
John Mann
1972 S3 Jaguar E-Type 2+2
1976 Jaguar XJ4.2C
2007 Jaguar XKR Portfolio

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42south
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#5 Re: Mystery vacuum port

Post by 42south » Wed Jun 14, 2017 2:03 am

Hi John
The advance or retard module is the silver unit on the lower side of the distributor. If it is operational it is connected via a vacuum hose to the port on the carburettor that I mentioned. Many people have just blanked the port on the dizzy if the unit has failed.
You can tell if it is an advance or retard module by the position of the vacuum port on the unit. If its advance the vacuum port will be facing away from the dizzy towards the rear of the engine. If its a retard module then the port will be facing inwards towards the distributor.
There is quite a lot about the vacuum module/ advance or retard on the site, try a couple of searches.
Basically the retard module was used for some early cars to try and meet the US emissions regs. It has a negative effect on performance.
The advance module uses the carb vacuum at cruising revs to put extra advance in the distributor to improve fuel consumption and performance.
This is the reason that vacuum is taken from the carbs, and not a constant source, to allow a modulation of vacuum. The position of the port on the carb for advance will have virtually no vacuum at Idle, enough to operate the module at mid range throttle opening, and with wide open throttle very low vacuum again as you do not want too much advance at high revs.
I just realised that I didn't explain why the different port positions on the carbs were chosen. The retard tap is just on the engine side of the butterfly which gives full vacuum at idle, throttle closed, to retard the ignition in an attempt to limit emissions.
the advance tap on top of the carb is on the outboard, air filter side of the butterfly, to give little vacuum when at idle, but enough to operate the module at moderate throttle positon.
Hope that's as clear as mud.
Cheers
Last edited by 42south on Wed Jun 14, 2017 9:00 am, edited 1 time in total.
Mark Brown
1971 S3 Etype
Son: when I grow up, I want to be a pilot
Dad: You’ll have to choose.

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vee12eman
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#6 Re: Mystery vacuum port

Post by vee12eman » Wed Jun 14, 2017 8:12 am

Hi Mark, John,

I am pretty certain all V12E-types are vacuum retard set ups, as are S1 XJ12 cars. As Mark said, you can check by the direction of the take off port on the retard uni. Towards the distributor for retard, away for advance. XJS and other later V12s use an advance unit.

Whilst it is generally considered that blanking it off improves performance, there is a debate about how effective it is and whether it is wise,so I elected not to go down that route.

Another issue is that the units are for some reason fairly unreliable and even new ones can fail early, which is what happened with my replacement.
Simon S-Y
Series III FHC

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John_Mann
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#7 Re: Mystery vacuum port

Post by John_Mann » Thu Jun 15, 2017 11:06 pm

vee12eman wrote:Hi Mark, John,
...... Whilst it is generally considered that blanking it off improves performance, there is a debate about how effective it is and whether it is wise,so I elected not to go down that route. .......
Hmm,

Well I've checked my setup and it is as you have said a retard vacuum unit, connected directly to the vacuum port under the rear LH carb on the manifold side, which bypasses the thermostat switch as it has been permanately plugged.
Now I have been doing a bit of reading on the subject and it appears that the retard system was purely implemented no reduce vehicle emissions at the cost of poor fuel economy and general performance. The fact that it is not going through the thermostat switch, in my case, means that there is always vacuum retard at idle and light throttle, even when hot.
It would therefore appear that the best option would be to remove this vacuum retard and plug the outlet to achieve improved fuel economy and better performance. I assume removing the retard vacuum shouldn't affect the initial timing in any way which should be at 4 deg ATDC, at idle?
There also appears to be an opinion by many to install a vacuum advance unit, but this seems to be a bit more complex as you need to get the right amount of advance and may also need to rematch the mechanical advance of the distributor and change timing accordingly. As to the benefits of all that, it is up for some debate.
I guess for now I'm probably thinking to disconnect the vacuum retard, which can only improve and not degrade in anyway, if my reading is right?

Cheers;

Image
John Mann
1972 S3 Jaguar E-Type 2+2
1976 Jaguar XJ4.2C
2007 Jaguar XKR Portfolio

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42south
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#8 Re: Mystery vacuum port

Post by 42south » Fri Jun 16, 2017 9:45 am

John
I use 12deg before tdc, for idle timing with the vacuum unit disconnected. This is specified in the service manual under the electrical section.
Do check your dizzy has movement and kick back when you turn the rotor a little way. Many get seized up internally from lack of lubrication down the centre of the shaft. Kirbys book available free to download has lots of good info
I ran my car with the retard disconnected for a number of years with no obvious ill effects, before going down the advance module route. You can just get a CEI dizzy from an XJS together with the electronic control box and drop it straight in. The timing can stay as is.

Cheers
Mark Brown
1971 S3 Etype
Son: when I grow up, I want to be a pilot
Dad: You’ll have to choose.

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John_Mann
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#9 Re: Mystery vacuum port

Post by John_Mann » Fri Jun 16, 2017 10:28 pm

42south wrote:?.......I use 12deg before tdc, for idle timing with the vacuum unit disconnected. This is specified in the service manual under the electrical section ......
Thanks Mark,
Oddly, my service manual shows the static timing as being 12deg BTDC and 4deg ATDC at idle. :scratchheadyellow:

I'm also running a new pointless distributor now with a Crane Fireball XR3000. Great system!
John Mann
1972 S3 Jaguar E-Type 2+2
1976 Jaguar XJ4.2C
2007 Jaguar XKR Portfolio

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42south
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#10 Re: Mystery vacuum port

Post by 42south » Sat Jun 17, 2017 4:24 am

Oddly, my service manual shows the static timing as being 12deg BTDC and 4deg ATDC at idle.

That's very interesting, Ive never heard of that before. Is it something to do with having the retard unit connected, although I understand that it only puts in around 8 degrees of retard.
My manual is the blu/black series 3 etype one E.165 issue1.
Mark Brown
1971 S3 Etype
Son: when I grow up, I want to be a pilot
Dad: You’ll have to choose.

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MarekH
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#11 Re: Mystery vacuum port

Post by MarekH » Sat Jun 17, 2017 5:52 am

There is nothing mysterious at all. Just add on the centrigfugal advance as stated in the repair manual at 86.35.00/3. (rememberering that 1 distributor degree equals 2 crank degrees)..

EDIT:- ....and the static number is with the vacuum disconnected

The parts manual shows how the various vacuum connections changed over the life of the car.

kind regards
Marek

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