Series III vacuum issues that really suck

XJ
1

Is my Jag “listing” toward Ported or “headed” for Manifold destiny?
I know not from whence my dizzys vacuum cometh,
As the delay valve is to me an enigma.

I found the output (“dist” nipple of the delay valve) produces 18"Hg (correction!!; 6"Hg) vacuum at idle on both of my similar cars (84 and 86 xj6). both manifold and ported vacuum are connected to the delay valve (“delay” and “carb” respectively). Any significant amount of vacuum at idle is manifold vacuum. at idle this vacuum advances the timing about 9 degrees so 6 to 8 would, I believe, be the basic timing setting (8 + 8 = 16 deg. at idle

However, both my original owners manuals (an 84 and 86 xj6)
specify 17 degrees BTDC as the basic timing setting. This is a setting for a vacuum advance using ported vacuum which should be 0" Hg at idle and hence no advance at idle.

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Is my delay valve faulty or my vacuum hoses routed wrong or what?

2

[quote=“00ring, post:1, topic:433414, full:true”]
Is my Jag “listing” toward Ported or “headed” for Manifold destiny?
I know not from whence my dizzys vacuum cometh,
As the delay valve is to me an enigma.

I found the output (“dist” nipple of the delay valve) produces 18"Hg vacuum at idle on both of my similar cars (84 and 86 xj6). both manifold and ported vacuum are connected to the delay valve (“delay” and “carb” respectively). Any significant amount of vacuum at idle is manifold vacuum. at idle this vacuum advances the timing about 9 degrees so 6 to 8 would, I believe, be the basic timing setting (8 + 8 = 16 deg. at idle

All basic ign advance settings are to be with vacuum disconnected, Scott! (Even with ported vacuum - though basically void…)

As said; with ‘manifold’ vacuum, the common centrifugal setting is between 4 and 6 deg. The list of distributors used by Jaguar for the xk engine is almost endless - though basically the engine itself, within each displacement version, requires the same advance curve, centrifugal and vacuum, for max power.

However, the definition of ‘best’ performance reflects different market priorities - traditionally the ‘European’ philosophy was max power. So the ‘Europeans’ used ‘manifold’ vacuum, with no regard to emission criteria - and originally specified advance for 98 octane petrol.
Different market priorities and octane variants used different basic settings and sometimes alternative distributors with different advance curves. But basically, all xk engines will run on any distributor - though more or less ‘perfectly’…:slight_smile:

Indeed, this is the standard US set-up, probably(?) related to emission control factors, and works OK, of course. But you cannot mix the two - usually there are spigots for both, though somewhat market dependent.

Ie - with ‘manifold’ vacuum; you set 4 or 6 deg, vacuum disconnected. With ‘ported’ vacuum; you set 17 deg, vacuum disconnected (just to make sure that setting is with centrifugal advance only). No ‘ifs’ or ‘buts’… :slight_smile:

The delay valve is not used with ‘manifold’ vacuum - yours may or may not be defective; it’s simply counterproductive with ‘manifold’ vacuum.

Crudely; ‘manifold’ and ‘ported’ vacuum varies in different ways with throttle position, throttle movement, rpms and engine load - hence the delay valve on the latter…

Frank
xj6 85 Sov Europe (UK/NZ)

3

It’s been a long time since I dealt with this so my memory is fuzzy. But this thread has some good discussion that might help. it has remarks from Dick Maury…who is always worth listening to, IMO.

I might study it myself, to refresh my memory !

Cheers
DD

4

According to this post below the advance vacuum is neither fish nor fowl but a combination of both depending on manifold vacuum/pressure. It seems the manifold vacuum is dominant at idle and becomes ported under certain conditions above idle. So what initial advance is proper to use?
It seems there is agreement that a combination of 17 deg.basic adv and and manifold vacuum at the dist. is just way too advanced. So that still leaves me wondering how the owners manual that came with both my cars states the idle wo/ vacuum should be 17 degrees.

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Dick_Maury

Doug_Dwyer

Feb '18

The vacuum controller is not a delay valve but rather a controller that supplies a steady minimum amount of vacuum at idle. Above idle, it lets ported vacuum take over when the port is above 6 in of vacuum for the XJ6 and 11 in for the V12 HE. At full throttle, there is not much vacuum so it drops off. This extra advance is to give the engine more advance ignition timing at idle to run cooler and use less fuel. This unit is also used on some American cars as one of the ports is labeled “Delay”. This unit does not delay vacuum but that is where it is hooked up on other non-Jaguar applications. Easy way to check is to measure the vacuum at the distributor with the above mentioned specs assuming it is hooked up properly. Delay=manifold vacuum , Dist=distributor vacuum unit and Carb=Ported throttle (not direct manifold vacuum)

5

Mine ran great at 17º, FWIW

As I recall it was a revised spec; earlier USA market cars were more like 10º base timing or something like that.

I have a vague recollection of researching this change maybe 20 years ago. I think it coincided with adding the second cat converter. There might be some archived info; pretty sure it’s been hashed out before, a long time ago.

Cheers
DD

6

The factory spec is correct for the later XJ6/3. The early Series 3 was 4 degrees and did not have the vacuum controller. Then they went to 14 degrees with the controller adding timing advance by a controlled 6" of vacuum. This was about 1983. Then they upped to 14 and finally 17 for the 1985-1987 cars. This was with the vacuum disconnected from the distributor. With it hooked up, the idle timing came out to about 20-21 degreed BTDC. Additional advance was controlled by the ported vacuum and if greater than 6", you got more vacuum advance. At full throttle, the vacuum could drop and the vacuum advance would taper off. The distributors at that time had different rates of mechanical advance and the total advance was about the same for all of them. As mentioned, the V12 and 6 cylinder use different controllers with different idle vacuum levels. You should not mix them without doing other modifications to the distributor or initial timing. You should use the timing as indicated by the specs under the hood of your car.

1 Like
7

Again, Scott; you need to verify vacuum source by measuring vacuum at the distributor hose in idle. If it shows 18" Hg or thereabouts, delay valve omitted; distributor is fed manifold vacuum - and idle advance setting is 4 - 6 deg, vacuum disconnected. Or succinctly; you will have the same vacuum at the dist as at the fuel pressure regulator vacuum hose.

The source cannot change between ‘manifold’ and ‘ported’ vacuum. If you have ‘0’ in idle; the source is ‘ported’ - and advance setting is 17 deg, and you use the delay valve.

With steady(!) pedal; ‘manifold’ vacuum remains at 18" (verified at the fuel pressure regulator’s vacuum hose) - above idle; ‘ported’ vacuum depends on throttle position and rpms

Again; we are talking about initial advance setting - if the dist is working as it should; it will set correct advance according to load, rpms and pedal position…

Frank
xj6 85 Sov Europe (UK/NZ)

8

Good morning Dick,
Thanks so much for dispelling the vacuum of sound information I’ve been caught in regarding manifold or ported. Your answer is most helpful and concise on the subject. I’ll go looking for the specs. under the hood to confirm.

9

Good morning Frank, I can verify manifold vacuum source at the “delay” port on what seems to best be called the vacuum controller. I can also verify ported vacuum source at the “carb” port. It seems your advise would best apply to omitting the controller and choosing one or the other to connect to the distributor and then time initial advance accordingly.
I found a treatise on this “vacuum controller” on the old site that gives a out of car test procedure which I’ll follow to insure it’s working as it should:
https://www.jag-lovers.org/xjlovers/xjfaq/delayvalve.htm

Thanks again for all the help rescuing this car from it’s otherwise immanent manifold destiny of becoming a just another imported “parts car”.

10

[quote=“00ring, post:9, topic:433414, full:true”]
Good morning Frank, I can verify manifold vacuum source at the “delay” port on what seems to best be called the vacuum controller. I can also verify ported vacuum source at the “carb” port. It seems your advise would best apply to omitting the controller and choosing one or the other to connect to the distributor and then time initial advance accordingly.
[(/quote]

I personally swear by vacuum advance, Scott - because I fully understand it…:slight_smile: But it is absolutely essential that you pick one or the other. You simply cannot use ‘manifold’ specs for a ‘ported’ vacuum source - or vice versa! They are different beasts…:slight_smile:

The ‘delay’ cannot increase the vacuum delivered from the source - it can only regulate how much is applied to the dist. It is inappropriate and counterproductive on the ‘manifold’ set-up…

Manifold vacuum dictates cylinder fill, at 18" Hg it’s 40 % of ‘0’ vacuum, and mixture temp increase with cylinder fill in addition to burn temps. At ignition the burn spreads from the plug at a speed of some 40 m/s - and ideally; and burn should be completed a tad after TDC for max power. Which means increasing advance as rev rise to give adequate burn time.

However, if ign is too early; the combination of compression and burn heat may rise mixture temp to the ignition point of petrol before TDC - the remaining petrol then spontaneously ignites, detonates; the engine knocks.

As cylinder fill relates to compression temps and the fill relates directly to manifold vacuum; any drop in vacuum increases temps - using vacuum as an advance regulation is obvious. At cruising speed vacuum is high, as pedal is pushed, vacuum drops and ign instantly backs off - as directed directly to the dist.

The point is to maintain high advance at light loads, near ideal complete burn point without encountering knocking at high loads.

‘Ported’ vacuum varies with air speed through the throttle and the throttle position - and differs from ‘manifold’ vacuum in a less obvious/predictable (to me) way. So a delay valve is commonly used to mitigate negative effects on the advance…

In early days in Europe, at motor tax was introduced based on engine ‘hypothetical’ horsepower - by engine volume. And huge efforts wwere used to get the maximum power from a minimum engine. And ‘manifold’ vacuum advance regulation was considered best. The US, without such tax, increased engine volume for more power - to put it very crudely. And used ‘ported’ vacuum to cover other priorities. Generally, and for various reasons, Jaguars on US specs has less power…

It is more complicated than that, and either set up works well enough, but has different priorities. Generally, the most efficient use of petrol is to run the engine as close to the detonation point as possible - using computers to regulate advance. And fitting antiknock sensors as a back-up…

Frank
xj6 85 Sov Europe (UK/NZ)

11

Hi Frank,

If I understand your advice: I should pick either manifold or ported and time accordingly. I should therefore take out the “controller valve” and plug off the vacuum source I choose not to use. Clearly both ported and manifold vacuum are now connected to this controller valve and somehow both together affect the single output that is connected to the dist.

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The larger hose on the left is manifold and the smaller ported, The hose on the right goes to the dist.

I put my basic timing at “stock” as labeled under the hood: 17BTDC, and found the vacuum advance took it to about 20 degrees at idle. The idle increased about 200RPM and was much smoother at this setting. have yet to road test it thoroughly.

Which source do you use for your jag?

12

This is somewhat bewildering, Scott…:slight_smile:

If you are on plain ‘manifold’ vacuum; using 17 deg centrifugal (dist vacuum disconnected) is plain wrong. That the vacuum advance, from 17 deg increased it to 20 (+3 deg) implies that the vacuum was ‘ported’ - and is consistent only if the ‘control valve’ is ‘doing something’

Questions; in the test - was the ‘delay’ valve connected as shown in the picture? What is the respective vacuum on the two ‘input’ hoses to the valve? (Should be 18 and ‘0’ respectively). What is the vacuum on the hose at the pressure regulator, the ‘true’ manifold vacuum?

Main question; was this test configuration used when you encountered the problems related in your first post? If not; indeed do the driving test. If that shows improvement/normal engine response; the ‘control valve’ is now working - which it may not have previously.

Of course; if the engine preforms as it should you may continue to use this set-up. But if you want to ‘convert’ to ‘manifold’ vacuum; connect the hose at valve showing ‘manifold’ vacuum directly to the dist - and set centrifugal advance to 4 - 6 deg.

Which of course you can do as a test - test driving the car in this configuration to compare the results…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

13

The plain manifold vacuum is being “controlled” to just 6" of vacuum at idle, according to Dick’s post above, and this matches what I measured. Hence, only a small 3 degrees or so of vacuum advance at idle.

Blockquote
Dick’s post:
The factory spec is correct for the later XJ6/3. The early Series 3 was 4 degrees and did not have the vacuum controller. Then they went to 14 degrees with the controller adding timing advance by a controlled 6" of vacuum. This was about 1983. Then they upped to 14 and finally 17 for the 1985-1987 cars. This was with the vacuum disconnected from the distributor. With it hooked up, the idle timing came out to about 20-21 degreed BTDC. Additional advance was controlled by the ported vacuum and if greater than 6", you got more vacuum advance.

Blockquote

I’ll recheck the vac. inputs and fuel pressure reg. All has become a bit muddled added to the fact that I misread the scales some of the time on the vacuum gauge (lens old scratched and foggy).

I did use this “controller/delay valve” set up when my problems started with the exception that i was at 6-8 degrees initial as I thought that was the normal setting (17 seemed against all my prior knowledge of old car initial advance settings)

14

manifold vac is about 16" and so is the fuel pressure source. The “controller” output is about 5" at idle. My gauge is old and was not a expensive one so may not be very accurate.

15

It likely explained your on-road problems, Scott - a tet drive will confirm…

To elaborate; for every combination of engine load and rpm there is an ideal point of ignition for max power. This is an inbuilt characteristic of the engine and cannot be altered.

The task of the distributer is to hit this point, but as both centrifugal and vacuum advance is mechanical analogue - the precision is pretty poor. The advance curves for both are fixed - though may vary with distributor type.

The only adjustable factor is the initial centrifugal advance setting - establishing a reference point somewhere on the advance curve. A main concern is that, due to the unavoidable imprecision, is to avoid detonation, pinking, is to initially set the dist at a position that avoids pinking.

Pinging onset varies with many factors, including petrol quality - and other priorities may interfere in decisions. Hence the variations in initial advance settings - and the different choices of vacuum source and controls.

That said; your vacuum is a tad low, indicating less engine power than optimal - but not alarmingly so. In general, as individual engines may vary due to degree of wear etc; it is perfectly permissible to experiment with initial advance settings.

Importantly(!); it is essential to test drive to verify that there is no prolonged pinking at high loads. There is no way to verify advance while driving. but as the engine itself works best as close to the pinking point as possible; no pinking at any time is not informative…:slight_smile:

So…?

Frank
xj6 85 Sov Europe (UK/NZ)

16

Thanks Frank for all your help. I’ll take your advice and move up the advance based on engine performance and back off when pinging occurs more than just very rarely and never prolonged.
That seems like the logical last step on ignition timing when the basics are all in order. As you say there are too many variables otherwise.

17

Indeed, Scott - it works equally well for either vacuum configuration. The two ways of avoiding pinking are either backing off or using higher octane fuel…:slight_smile:

As an aside, as mentioned before (and referring to your experience when increasing your advance) a sometimes method is to finely tune ign timing for maximum idle manifold vacuum and rpms - vacuum connected to the dist ,of course. Test driving is mandatory with any experimental ign timing adjustment…

Frank
xj6 85 Sov Europe (UK/NZ)