Vacuum advance leaking

(V12 Dave) #1

I’m in the process of going over the distributor in my '87 XJ12 (Cdn digital P) and checked the vacuum advance as noted in the manual.
with a hose attached and my best effort at vacuum by sucking on the other end two things happen (or don’t).
The advance doesn’t work and I can hear leakage in the advance, same if I blow on it so it appears that the diaphragm is leaking.
Quick check on my usual suppliers came up empty.
Does anyone know of a source for these or if a re-build service exists?
Thanks all, and Happy New Year!

(Robert King) #2

British Vacuum is best; although I bought mine from SNG Barratt. First one I received from SNG leaked, but they replaced under their part warranty. The SNG Barratt was slightly different, it did not have the limit adjustment and the vacuum nipple was oriented differently and fouls a spark plug boot. The offers the limit adjustment.

(V12 Dave) #3

thanks very much Robert, I am going to contact them tomorrow

(Aristides Balanos) #4

Mind you, that the original vacuum advance unit for the Digital P has a little hole on the diaphragm.
This is necessary as the dump valve and vacuum regulator need some flow to work correctly.

This is how I fixed mine…


(Robert King) #5

I do know that the one fromSNGBarratt does not leak vacuum (that is until it fails). The first I received held vacuum when installed, it failed a few hundred miles after installing, with a noticeable difference in performance. The second has been fine for 2500 miles (I just checked it Saturday).
There would be no need on mine for a bleed, as the vacuum dump vents to atmosphere.

(V12 Dave) #6

Thanks for your post Aristides, I was not aware of the hole.
I do still suspect the diaphragm as I could not get any movement out of the lever regardless of how hard I sucked on the tubing attached to the vacuum inlet.
Also the car was running very poorly when I shut it down for all the work I have been doing, and while poor performance could be due to a number of things, it starts to make sense.
With a hole in place and everything fixed would you expect to get movement from the suction applied by sucking on the tubing to the inlet?


(Aristides Balanos) #7

Yes you should get movement.
But it need a strong vacuum, a vacuum pump or a big syringe.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #8

Nah, you should be able to easily suck in a few degrees of advance by mouth. If you have the orifice type, while sucking, you can put your tongue on the end of the tube and the advance will bleed off in a couple of seconds. With the non-orifice type, you can put your tongue on the end of the tube and it’ll hold the advance indefinitely. If the diaphram is ripped to shreds, you can easily suck air through it and the advance doesn’t move.

(V12 Dave) #9

Thanks for the responses guys, I am quite sure the unit is not working correctly and am arranging with British Vacuum Unit to have it rebuilt.

(Paul M. Novak) #10

I have found it very helpful to own a small hand held vacuum pump, like a Mityvac, for a variety of test purposes on my Jaguars. Not only is it useful in checking distributor vacuum advance, but I have also used it to test fuel pressure regulators, transmission modulators, draining brake fluid from the reservoir, siphoning coolant, checking the EFI ECU and cruise control bellows for vacuum leaks and more.

The only drawback that I can think about using a Mityvac over mouth suction is not having the taste of automotive products in my mouth. :wink:

Mityvac cost about $40 US. I have no affiliation with Mityvac except as a satisfied customer.


(V12 Dave) #11

Hi Paul

Thanks for the note and the suggestion, part of the fun of the hobby is acquiring the tools!

I am going to look for a Mityvac, sounds a lot better than the end of a dirty hose in the mouth.



(Paul M. Novak) #12

My younger brother and I are having a contest, “He who dies with the most tools wins”. I am pretty sure I have the most tools, but it is close for sure. :wink:

The Mityvac has a built in vacuum gauge and you can use the hand pump to set a specific vacuum and then watch over time to see if you have a slow leak in a component or system. This is a good way to catch small leaks or observe if a designed in bleed port is working properly. Mouth suction, regardless of how good it is, is not calibrated. :wink: