Jaguar XJ-S. Service manual — part 33

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plug hole than the 1A. And, obviously, there are many possible variations on this idea, including threading a fitting
into the spark plug base and the use of a balloon instead of soap. Note that this method won’t work if you’re at the top
of the exhaust stroke instead of the compression stroke, so if you seem to be having trouble you might want to give the
crank one complete revolution and try again.

The ignition timing on the V12 is checked with the engine held at 3000 RPM. Although a pain, this method insures the
timing is accurate at operating speed rather than at idle, where timing is less critical. However, proper timing by this
method requires that the tachometer be at least reasonably accurate. It’s not very critical, since it would have to be in
error by about 300 RPM to cause an error of 1° in the timing. Nevertheless, if one ever finds his car hooked up to one
of those fancy, super-accurate computerized engine analysis machines, it is suggested that the location of the
tachometer needle be noted when the engine is running at a real 3000 RPM. That point can then be held whenever the
timing is checked in the future.

When checking the ignition timing on the V12, the vacuum line to the distributor vacuum advance must be
disconnected and plugged. The vacuum advance capsule is at the bottom rear of the distributor, making it quite
difficult to get to this line (the larger hoses connecting to the distributor cap itself are for the distributor ventilation
system, and do not affect timing). Do yourself a favor and cut the vacuum line somewhere convenient and reconnect it
with a small piece of tubing. From then on, all you have to do is disconnect it at the break and plug it when checking
the timing.

Since the battery is in the trunk, connecting the power leads of the timing light requires ingenuity. The ground lead can
be connected anywhere on the car. The positive lead must go to 12V, which exists at any solid brown wire. There are
two terminals on the firewall adjacent to the valve covers, and a terminal on the back of the alternator -- all difficult to
get at. Another solution is to use a pair of jumper cables from the battery in the trunk. Peter Smith: “I connect my
timing light to the 12V power source at the headlights fuse box.”

Since disconnecting the plug wire from cylinder 1A is difficult, disconnect the wire from the distributor cap instead;
make a small jumper from an old ignition wire to use to connect the timing light. Or, just buy an inductive timing light.
If it’s more convenient, you can also check the timing using the signal from cylinder 6A.

Since you must crawl under the front of the car while an assistant holds the engine at 3000 RPM to read the timing, it is
suggested you put an extra long ignition lead on your timing light so you can route it around the fender rather than
feeding it through the engine compartment. Interference with moving parts at 3000 RPM would be memorable. Cheap
ignition wire with metal conductor works well for making a long timing light ignition lead, and is available by the foot.
Steve Chatman came up with another idea: “When checking the timing on the XJ12, I discovered that I was either
going to extend the wire that clamps the plug wire or feed the light through the engine compartment. I didn't really like
those alternatives and instead simply ran the timing light over and down the outside right front wing. From there, I
could shine the light under the car, using my left hand. With my right hand, I held a mirror to reflect the light vertically.
Worked fine and seemed very safe. Of course, you should correct for the increased distance traveled by the light, but
you knew that. I was in a poorly lighted garage at night. Better lighting could make this approach difficult or
impossible to use.”

Michel Carpentier provides another way of dealing with the inconvenient location of the timing marks: “Make a
pointer out of sheet metal, which you fit under one of the water pump nuts (pointing from above towards the crank
pulley). Now position the crank so it is in firing position at 3000 RPM and put a dab of white paint opposite your
pointer. From now on you can check your timing from above.”

Timing is adjusted using a long screwdriver to turn a small eccentric cam on the side of the distributor underneath the
cruise control actuator. There is a locknut on the adjuster, and this is a place where a crowfoot wrench will come in
very handy.

The Jaguar V12 design eliminates most wear items that would normally cause ignition timing to vary, such as ignition
points or sloppy camshaft drive systems. The only remaining reason for the timing to change would be wear in the
timing chain itself, and the Jaguar timing chain normally wears so slowly as to be insignificant. If the timing in your
car is way off, it is highly recommended you determine why rather than simply readjust it. For example: If your
centrifugal advance unit seizes in the idle position, then when the car is revved to 3000 RPM the timing will be retarded

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by 15° or more. If you merely adjust the timing rather than correcting the problem, then the timing will be 15° too far
advanced at idle!

If your eccentric cam timing adjustment won’t go far enough, the distributor base must be repositioned. Remove the
distributor cap and insert a long allen wrench to loosen the three mounting bolts at the very bottom of the unit. Rotate
the entire distributor housing in the direction needed, then retighten. Please remember that this much adjustment should
never be necessary, and causes should be investigated.

HOLDING A HIGH RPM: Michael Minglin sends a tip: “Every time I went to check the timing, alternator, etc. the
book says to set the rpm to such and such. Reaching down to that auxilary air valve when the engine is hot is not my
idea of fun. What I did was to weld a nut on top of a short bolt. Threading a longer bolt into this nut gave me a “T”
with a long top and short leg. The short bolt goes into the RHD throttle cable bracket (not used for anything on my ’84
XJ-S), Then I use the long bolt to adjust the bell crank to the rpm I want. To make it even easier, I forced a short piece
of vacuum hose onto the long bolt, at the head. This makes it easy to adjust the bolt with your fingers.” This plan will
work on either LHD or RHD cars, simply using whichever cable bracket is unused.

CENTRIFUGAL AND VACUUM ADVANCE DATA: When checking distributor advance versus tables, make sure
you are looking at the H.E. or pre-H.E. tables, as appropriate. The H.E. system develops less advance in the centrifugal
mechanism and more advance in the vacuum mechanism than the pre-H.E..

Page 05-1 of the ©1975 ROM and page 05-1 of the ©1982 Supplement list data for the centrifugal advance
mechanisms for the pre-H.E. engine. Both of these charts -- as well as the centrifugal advance data on page 112 in the
Haynes manual -- begin with a listing that says “No advance below 900”, which would seem to indicate that the entire
chart is talking about the amount the advance changes from idle or static settings. However, Roger Bywater points out
that this is not the case; the other entries in these charts are actual crank degree readings based on the proper 10° BTDC
baseline timing at 500 RPM. For example, the chart in section 86.35.29/4 says that the distributor should advance 6.0-
8.0 degrees at 1000 RPM; since these are distributor figures, that corresponds to 12.0-16.0 degrees at 2000 RPM at the
crank, and when added to the 10 degrees of baseline advance we get the 22-26 degrees of advance listed in the charts.
The charts would have been clearer if they had simply said “10° below 900” instead of that “No advance below 900”
stuff.

Regarding the charts for pre-H.E. advance, Mike Morrin says, “I suspect the manual only has data for one of the
distributor variants fitted (probably C44663). The parts book shows that there were 5 different models of distributor
fitted to pre-HE XJ-Ss, and I think that the only difference was the advance curves.

C.43735

Australia (this is the one with the peculiar vacuum retard system).

C.44663

California to car 2W54183

C.43735

California from car 2W54184

C.44663

CDN/USA to engine 8S5461 (ie the 4460th engine what year??)

All other countries to engine 8S5202

C.46173

CDN/USA from engine 8S5462

All other countries from engine 8S5203

DAC1609 California from engine 8S11161 and Australia from 8S11800
DAC1380 All other countries from 8S11262

“Note that my parts book does not cover the [pre-H.E.] Digital-P cars, so there are probably 2 more I have not counted.

“On reflection, I am surprised that there is not a clearly different distributor model (advance curve) for 8:1 and 9:1
compression engines. As far as I can see, the distributor seems to change according to degree of emission control, not
compression. For example: Australia (9:1) and California (8:1) get the DAC1609, all other countries presumably
including the UK (9:1) and the rest of the USA (8:1) get the DAC1380.”

For our information, Morrin also provides part numbers for the distributors on the Series III E-Type:

C33148

up to engine 7S4663 (emission control) or 7S4879 (no emission control)

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C37443 from

above.

“The carburetted distributors of course have no FI trigger assembly.”

TIMING THE PRE-H.E.: The procedure is described in the ROM, section 86.35.29/7. Basically, you disconnect the
vacuum advance, lower the idle to make sure you’re off the bottom of the centrifugal advance curve, and set the timing
at 10º BTDC. Much easier than the H.E. procedure!

Unfortunately, there’s probably a very good reason the H.E. went to setting the timing at 3000 RPM instead of at idle.
Roger Bywater speaks of distributor build quality problems: “Back in the early 1980's we encountered V12 distributors
which could over advance beyond spec by as much as 10 degrees at 6000 revs!” What this means is that with the
timing set correctly at idle, it may still be way off at highway speeds. You’re not likely to burn pistons at idle; it is far
more important that the timing is correct at highway speeds. The H.E. procedure minimizes the implications of the
problem: Even if the distributor isn’t quite within tolerances, the timing will be correct at 3000 and will merely be off
at idle instead. Much safer.

So, why don’t we adapt the H.E. timing procedure to the pre-H.E.? That way, those cars will enjoy proper timing at
highway speeds as well. Bywater: “What you are saying is absolutely correct and setting the timing at low speed
before any advance takes effect is just about the worst way of doing it, but to be honest I've never really thought much
about it before. In all normal operating conditions the advance could in theory be 4 degrees out yet still be within spec.
Allowing for wear and tear, not to mention questionable original build quality, the error could easily be more than
that.”

All we need to do is figure out what the timing should be at 3000 (or some other point in the middle of the curve
somewhere) and set it there. The fact is, we could provide a spec for timing at 2000 or 4000; it wouldn’t make much
difference. As long as it’s set somewhere along the curve rather than at idle where you’re not even on the curve, the
advance will be much closer to correct where it’s important than it would have been with the timing set at idle.
Bywater suggests that it’d be best to check the timing right at the peak torque, since this is where it will be most critical
due to maximum cylinder pressures. Ideally, the timing should be checked at several places to make sure none exceed
an acceptable advance, but let’s not get carried away.

To generate mid-curve timing specs, we can combine the idle timing specs with the distributor advance mechanism
specs. The specs in the ROM for checking the distributor advance mechanism provide a 4º tolerance range. A 4º
tolerance band may be good enough for checking to see if the distributor is working but it’s not good enough for setting
the timing; we need to know what point within these tolerances is the optimum timing point. We could assume that the
tolerance is ±2º and specify timing based on the midpoint, but Bywater suggests this might not be right. “With regard
to the tolerance spread it is worth remembering that all engines must be safe with the worst possible condition which
implies that the most advanced point is the one to work from rather than mid-range. I am not sure how tangible the
benefits would be but if it makes it possible to run safely with 2 or 3 degrees more advance then the overall
performance, response and economy should be improved. I am sure it is worth trying but maybe a period of trial is
called for with a few willing volunteers who can report back before advocating it on a wider scale.”

Bywater went on to provide timing specs for all pre-H.E. V12’s:

Carburetted V12’s

35° BTDC @ 4000 RPM

D Jetronic:

33° BTDC @ 4000 RPM

10:1 CR pre-H.E. Digital P

24° BTDC @ 3000 RPM

Are these correct? You just can’t get a better authority than Roger Bywater; he was the guy leaning over a screaming
V12 in an engine testing room at Jaguar dialing the advance up and down by hand to develop these advance specs. But
anyone can make a mistake. So, after you set the timing using these guidelines the first time, check the timing at idle.
If it’s only different by a couple of degrees -- notably if it’s more advanced by a couple of degrees -- the spec was
correct, get in the car and drive. If it’s off by ten degrees, check to see if your advance mechanisms are working
properly. If they’re OK and the timing is really that far off, perhaps it’d be wise to rethink this whole idea before
putting your foot in it.

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Yes, the numbers for the pre-H.E.’s sound like a lot more advance than the H.E.; that’s the nature of the beasts -- the
H.E. gets less advance from the centrifugal mechanism and more from the vacuum capsule than the pre-H.E.

So, for those unfamiliar with timing the H.E., here’s the procedure: Warm up the engine, disconnect and plug the
vacuum advance, have an assistant rev the engine up a little past the specified RPM and then come back down to it and
hold it there, and set the timing to the specified advance.

Of course, one problem is that the scale on the timing plate doesn’t go far enough. So, set the crank at 20° BTDC and
paint a new mark on the damper at 0°. Then you can use this new mark to set the timing at the spec minus 20°.

UNDERSTANDING THE VACUUM ADVANCE SYSTEM: Between August 1998 and June 1999, Kelsey
Publishing (page 721) published six issues of a magazine called XJS Bulletin. Issues 1, 2, 3, 4, and 6 contain a series of
articles called “Advancing by Vacuum” by Roger Bywater of AJ6 Engineering (page 710) that are arguably the best
explanations of the vacuum advance systems used on the Jaguar V12 available. For those interested in obtaining back
issues, here is a brief synopsis of what each issue’s article covered:

Issue 1: General concepts of vacuum advance systems

Issue 2: 1976-80 V12 vacuum advance systems, including California and Australia

Issue 3: 1980-81 vacuum advance systems (pre-H.E. Digital P cars), Emission A and B

Issue 4: Advance concerns related to the H.E. engine

Issue 6: H.E. vacuum advance systems, Emission A and B

After June 1999, XJS Bulletin was discontinued as a separate publication and a distinct section of Jaguar World
magazine was provided instead, with pretty pictures of cars but little in the way of useful technical information.

VACUUM ROUTING DIAGRAMS -- WHICH IS WHICH? Good question. The following is an attempt to clarify
the applications of diagrams that appear in several documents; here’s hoping I don’t make any errors here, since that
would really compound the confusion!

In the ©1975 ROM, there are no vacuum routing diagrams. For the vast majority of cars of the time, the routing for the
vacuum advance was simple: Straight from the throttle edge tap on top of the right side butterfly housing to the
distributor. The air pump, where fitted, was apparently uncontrolled -- on all the time. In California and Australia, the
system was more complicated, but not covered in the ROM.

The ©1982 Supplement has two descriptions, one labelled “Emission B” and the other labelled “Emission A & C”.
These both apply to the H.E. only, but the Emission A & C version only applies through 1982; for 1983, a vacuum
regulator was added to the system.

The H.E. supplement in the back of the ©1984 Ed 4 includes a desccription of a system labelled “Emission A & C -
Federal, Canadian & Japanese Specification 1983 on” which includes the vacuum regulator omitted in the ©1982
Supplement.

The Haynes manual (©1986) actually contains a fair assortment of vacuum routing diagrams, enough to put the ©1975
ROM and ©1982 Supplement to shame. Of course, they’re not labelled clearly enough, so here goes: Figures 13.29
and 13.30 apply to the 1980-81 pre-H.E. engine with Digital P injection. Figure 13.31 applies to the pre-1983 North
American H.E. Figure 13.32 applies to the 1983-on North American H.E. Figure 13.33 applies to the Emission B
(non-North American) H.E. Figure 13.33 is labelled that it applies to the Australian models, but it’s not the same
system that Bywater’s XJS Bulletin # 2 article shows for the 1976-78 Australian cars; presumably it’s for the later H.E.
models.