1985 xjs he vacuum advance module

Greetings Jag Lovers,

My vacuum advance module is well and truly cooked. Looking for tips if poss. on how to remove the module from the dizzy ? How is it attached ,do I need to remove the star wheel. Photo’s would be great.
Any help would be great.


Hello Bruce,

Best to remove the dist out completely, the vacuum advance is held in place with a little spring pin at the dist body, quite difficult to do it in situ. A good chance to service and lube the centrifugal advance as well.

Don’t know if the advance capsule is available, but there is always the option to take it appart and change the membrane, albeit a bit more involved process…

I have to agree with removing the dizzy. I did so I could service the lot, I pulled down the mechanical advance and did a rebuild. Well worth the effort. I was able to purchase a new vacuum unit but that was 6 years ago.

yes, pull the distributor. Quite easy on the bench. The little pin that needs hammering out/in would probably be impossible on the car.

Also, I would recommend having yours rebuilt if it’s original. I bought a new one, only aftermarket available. It was so cheap compared to the original, so I had mine rebuilt by BritishVacuumUnit. Good as new. $80.

Later vacuum modules have a built in leak. Mine moves with a hand vacuum pump, but leaks down as I guess it’s supposed to. How the heck do you know if it meets spec or not?

If you can’t pump fast enough to move it, it doesn’t meet spec.

Well, I guess it’s ok then since I can get it too move. What a dumb way to control advance. They’ve got so many other valves, dumps, and limiters in the vacuum line, it’s a wonder it works at all.

The deliberate leak is necessary to allow the vacuum regulator to work as intended; it won’t work right on a fully sealed line. But yeah, I agree with you, having a vacuum regulator in the line – which resulted in the need for the vacuum dump valve as well as the leaky advance module – was a case of overthinking. It might have been the only way they could come up with to meet emissions, though.

Y’know, you can scrap all that stuff and just connect the vacuum advance to the port on the RH throttle body. It’ll idle a bit slower due to being a bit more retarded at idle, but the advance will be the same at all other throttle points.

Thank you all for your input,

I will take your combined advice and pull the dizzy, give it a good service,and replace the vacuum module with a good rebuilt second hand one that I managed to get hold of. If I mark exactly the present position of the dizzy and replace it the same spot, do I need to reset the timing?



aaaand, 20 characters.

When I rebuilt mine I chose to have the vacuum unit fully sealed and instead installed a leak valve thingie somewhere in the vacuum line.
I tested the car with and with out the leak device, it was easy, but found no difference on the idle whatsoever…
Maybe the regulator is bad ? don’t know.
I guess I should connect a vacuum gauge and take readings with and without the leak.

We’ve talked this one over and over…one thing to remember, retarding the idle a bit will result in higher engine temperatures at idle, right?

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Yes, but if it’s a problem, your cooling system needs work.

so what is the preferred method? My complicated vacuum advance system is working fine, but that three way valve and blowoff valve, I don’t know how long they will last. I guess I’ll just leave it, or does your method give slightly better idles but with less fuel efficiency?

If the system works, might as well leave it. It’s when something goes bad that the option of doing away with all the extraneous hardware starts to look attractive. I don’t think it’d have a significant bearing on idle, fuel economy or overheating.

Now that i’ve got my V12 idling smooth and running top, i decided to try that hookup again.

I had to turn idle screw out one full turn to get back to correct idle rpm.

First impressions, the idle sounds much deeper from exhaust pipes (i have no cats, no mid mufflers, only rear mufflers). And i swear it idles smoother and the throttle response at lower throttle is more linear with no lag if that makes sense.

Now, my timing @3000rpm is 22 BTDC instead of 18, because i run 92 octane instead of required 87. (USA) So maybe the stock 8inHg (or whatever that three inlet valve supplies to vacuum advance at idle) supplies too much advance at idle for me?

I’m going to keep it like this for a week and see if i like it. Less junk!

Second impressions…car idles a bit worse when cold, and throttle response is bad when cold. Once warmed up, idle is fine, maybe same as before, maybe better, hard to tell. But throttle response when warm from a stop to flooring it not as good.

I’m going back to OEM setup!


Continuing the discussion from 1985 xjs he vacuum advance module:

Continuing the discussion from 1985 xjs he vacuum advance module:

Thanks for all the info.

I am not sure if my regulator works or not. When I get the replacement vacuum module fitted that will be the test. Not sure yet if the new unit has a bleed hole or not.
If the module is faulty I will try the direct hook up and take it from there.

Once again thanks for the help,

Check out British Vacuum Unit on the internet; they offer a “better” quality replacement than the usual aftermarket replacement (of which I had one fail within a few hundred miles). No affiliation…

Thanks for that Robert, I have just sent them an email with all my specs.