I’ve read things from grant, doug all the xjs greats.

The vacuum advance I’ve attached to the top port. Iv read “which i cant find now”.
that some attached to the bottom port.
Is there any difference between the two?
which is the more or less preferred way?

 

There is definitely a difference, i cant recall specifics. The throttle butterfly will change when it sees vacuum

 

Shawn,

Your emissions requirements will dictate the plumber, and that is VAGUE at best.

I use the Inner/Lower port. Meaning the port on the engine side of the butterfly, as apposed to the atmospheric side.

I only have one hose, port to capsule, and using the lower port means its unseen, and neater. I used to care about that, not so much now.

The Top port in our market was atmospherics from memory???

Vac diagrams on the HE are the most vague I have ever seen, so, clearly lots of changes to meet Emission needs for some markets. Hence I ALWAYS say, "One Vac Hose At a Time" when replacing them, your memory aint that good.

 

HA thx Grant, I have removed all the "emissions" vac lines a while ago thanks to your wisdom, iv kept the little brown vac regulator thingy to get some advance at idle, might just remove that. On our preHe coupe i just did the vac capsule to port as well.
I just didn't know if there is a huge difference when the vac become "engaged" between under the butterfly or the top one.
I also as of yesterday, just doing normal checks on the XJS, my Vac advance is dead, I guess after 9 years it gave up. Feels like I just did that and the fuel hoses, times flys sheesh.

 

AHA, time really flies as we age, dont like it , ha. Just took 4 hours to change the oil on the S Type.

9 years is a good run for that capsule.

Different ideas on that vac source for timing. I went with vac at idle and adjusted the timing to suit ME. I "drive time" them all.

 

There is diff between vac ports.

You want the port that always sees manifold vacuum, even at idle, for best performance and fuel economy. IF you don't have to pass US emissions tests.

The vac port above the throttle plates doesn't allow much vac advance at idle and low speed, which help reduces nitric oxide emissions, but hurts fuel economy.

Doug

 

Thx for the help guys. Im gonna be moving it to the lower port.

 

My advice is to keep the vacuum advance with the reducer connected as the factory had it, just eliminate the 45 second solenoid; here is why-
Direct connection of manifold vacuum will cause idle stability problems on engines (such as the Jaguar V12) without computer controlled idle. This is because the idle vacuum varies a lot on starts and with a/c compressor load. What happens is that the vacuum drop will cause the timing to retard, which causes the vacuum to drop further, etc. The vacuum reducer limits the vacuum advance to approximately 11” vacuum at idles, as the manifold vacuum does not drop below that under the conditions mentioned above.
But you do have to have a good vacuum capsule- I recommend British Vacuum Unit after having 2 replacement units failing within 2 years; they can custom build one for you with a HD diaphragm. No affiliation other than a happy customer.
BTW, the ports on the bottom of the right intake have vacuum at idle that diminishes as the throttle opens; the ones on the top have no vacuum at idle but are active after a certain throttle opening. Connect per factory.

 

I restored my vacuum system to factory spec, at some point the timer was bypassed. If all the parts work i dont see a reason the remove this stuff. Is there a downside to running it in factory configuration?

 

No downside that I am aware of. Only real detrimental system is the air injection system; all it does is provide air the the catalysts for 45 seconds on start (to get them functioning faster, also to fool the oxygen sensors to enrich the mixture for 45seconds). You can ditch that and save weight plus the power to turn it.

 

Canadian cars to 87 model year(mines an 86) don't have the air pump thankfully. Only 1 cat per bank too.

 

Obviously we were blessed downunder.

NO cats until late 1988 Build Date.
Very little emissions "stuff" until about the same time.
We dont have emission testing, MOT, whatever, so, again, very blessed.

One of my May 85 Build cars, had an air pump, and it had failed, so removed it and the junk.

Remember. this engine was designed and spec'd in 1954, different world back them. It came with 4 Carbies, NOT in the XJS, and did what it did very well.

The emission choking started with different markets requiring all sorts of results from a tube up the cars rectum. Some worked, some failed, most "killed" the true spirit of the V12, and the 6cyl variants of the time as well.

Sadly, with the introduction of sensors etc 'talking" to the ECU for exhaust gas sensing, there is little that can be done to ease the "mess" in the car. Availability of some of that "stuff" is now near zero, so the car becomes un-roadworthy in some markets, very sad.