What is the compresstion on an XJ6 motor?

Hi Jochen
Which advance curve do you use on the 123 distributor? From the information given with the kit, I can’t find any numbers on the distributor that match thier data?
Thanks Nigel

Over the years a few have, yes. Can’t hurt.

New coils, purchased withing the last few years?

Ages ago I used to specifically seek out Lucas brand ignition parts because they were good. However, it seems that since Lucas sold off its automotive division (15 years ago?) the ignition parts seem to be lower grade. Some of the other stuff, though…like switches…seems to be better.

Coils have been a slight bugaboo on these cars but it was typical to get at least 8-10 years out of a Lucas coil, or more.

Out of curiosity what part number were you getting? Some of the parts listings are incorrect on the coils.

Cheers
DD

Sorry Nigel,

as it has never given any trouble (knocking wood), I’ve never touched
it, and the PO hasn’t left a note how he set it. So I have no idea what
curve the engine is using.

But keep me posted with your experience:-)

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec.)

I’m not sure, That was two years ago, and I’d have to look I purchased them through partsgeek.com

It was this one
[//jl-discourse-uploads.s3.amazonaws.com/original/2X/c/ca16c1a684c318fa69d8ad4b35db0e36c837e200.png]
Needless to say the notes didn’t give me warm fuzzies. I ended up with a MSD Blaster 2 coil. Naturally it says to mount upright because it’s filled with oil. That is what prompted the coil relocation post.

**
While the xk will run happily on ‘almost any’ coil, Doug - the points system basically should use a ‘6V’ coil with external resistor…

There is some worry that ‘high power’ coils may allow to much current through the AB14, causing failure. Most aftermarket prescribe bespoke coils with their systems - for that or some other reason…?

However, the main disadvantage of mechanical points systems is that they require ‘constant’ attention due to unavoidable points burning - which over time alter ign timing. One advantage of the electronics is unchanging timing and ‘no maintenance’ until, or if, they fail. A second is that the spark voltage is 2 - 3 times higher, though points voltage is perfectly adequate…:slight_smile:

And I sure do not advice changing electronic ignition for mechanical points…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**

This is where I relocated the coil on my ‘66 S type


Excuse the condition been in the garage for the last 20+ years you can see the expansion tank I fitted as well.

OK, seeing we have diverged on to points, on my motor, which has paperwork for a big $ rebuild, including “recalibrated” dizzy.

If I set points to between .012 - 014 tho, dwell is incorrect. (too low I think)

To obtain correct measured dwell of 30 to 35* points gap is down to less than .010"

(and that was how it was set when I obtained it)

S2 XJ6 motor, new points. I replaced this dizzy with a 420G for the moment, no noticeable change in performance, but points gap matches expected dwell

any ideas on what might cause that situation with the correct dizzy?

**
The ‘dwell’ angle denotes the angle of rotation that the points stays closed, Tony - which is the ‘charging’ phase of the coil; current flowing through the coil primary. As soon as the points starts to open, the primary coil power is disconnected - and coil secondary discharges high tension voltage to the plug in line with the rotor…

From this follows that if dwell or gap is altered; you also change ignition timing - requiring timing reset.

Basically, the correct dwell is important for spark energy - the gap ensures that the points open sufficiently to ensure that coil primary is disconnected. In principle, the points are configured to give the proper dwell with the correct gap set.

To elaborate; measuring dwell you measure the angle the electric points are closed - during this phase, the tip is not in contact with the cam lobes. As the engine rotates the cam lobes hits the tip - and the electric points separate. Measuring gap you adjust the electric points with the arm tip on cam lobe high.

Anomalies with the tip length or points wear/burning may interfere with the relationship between the gap and dwell angle…?

If the difference cannot be eliminated, you can chose correct dwell or correct gap - or something in between. In any case; if adjustments are changed; ign timing must be reset - and if the engine works as it should, it’s likely as good as it gets…

Frank
xj6 85 Sov Europe (UK/NZ)
**

Interesting that you saw no difference between the S2 dizzy and the 420G dizzy. The latter is IIRC virtually identical to that of the S1 E-type, both designed for 9:1 and triple HD8s.

The dwell angle-point gap relationship depends on the diameter of the cam I suspect, and might not always be correctly specified by Jaguar, since there were so many dizzy variants. Just a thought.

Just gives me a head ache, this early in the morning. Points and condensors1

I do have a dwell meter and feeler gauges from the old days.,

  1. Old Ford V8’s had an advanced distributor. As I recall, developed by Mallory for Henry. Mounted on the front of the engine and driven almost directly off the nose of the cam shaft.
    Slick, it only fit the “correct” as to TDC. Dual points ! Increased dwell. Hot spark. And the coil atop the distributor. One less place to corrode. Timing variable, only very few
    by a screw and a slot. Vacum brake on the advance.
    When all right, great, hot spark almost for sure.

Alas, point gap or replacement a bench job. At best, a Sun machine needed. AT worst, a complex manual method I never really understood.

Racers ,with this engine usually had two or more distributors preset and ready to go. Swapping on a hot engine no fun,. but doable… Ouch!!!

Carl

Tony,

maybe I missed something, but did you investigate the vintage of your engine? SII cars are supposed to have either Lucas 22D6 or 45D6. Contact gap is indicated at 0.014 -0.016, timing (static) @ 8° BTDC. Maybe someone put on a SIII head and/or different cams?

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

did get the points gap wrong as I nominated 12-14 instead of 14-16, but apart from that;

It is a rebuilt S2 motor, placed in my 420G, as the old motor was burning oil.

I did not know it was rebuilt when I bought it, but subsequently got the build sheets showing a high-quality rebuild

It had the correct dizzy for a S2 XJ, but I temporarily swapped the older 420G dizzy back in to see what was going on.

No amount of timing, or other adjustments will make the engine run as well as I want.

It has somewhat uneven idle, and there is a mysterious whistle emerging from rear carb, and I do not believe it is delivering as much power as it should

As the car drives reasonably well, and I have eliminated all the possible causes I can think of, my intended course of action down the track is to remove the cylinder head and examine all parts to find the cause of the whistle, fit the triple manual-choked HS8 carbs, and (hopefully) EDIS ignition

Id do a propane search by that carb: that is a fairly clear symptom of an intake leak.

done that, checked for all vacuum leaks repeatedly, and even swapped out the entire rear carb

there is a audible whistle at around 1000-1500 RPM

vacuum gauge shows a rapid flutter of ~2 degrees

applying tube & ear to each carb inlet, front one sounds very smooth

rear one, I can hear the flutter, it is the sound of air moving across a disturbed surface, a part that is moving

Its got me (and others) stuffed,

according to “vacuum gauge charts” the symptoms described are most likely
“worn or damaged valve guides or springs”

It passes a compression test 160psi on all cyls, tappets clearances in spec

That’s what Id go with.

I was going to change springs in situ, but have decided head off is the only way to check the guides, and am a bit worried about one letting go in the meantime

the part may not be moving, but in one of three rear valve train opening & closing cause the frequency

I can just tell something aint right

Hi Tony, i had an inlet guide slide down on an earlier engine long ago. similar symptoms but i did not do all the tests you did as i thought it was a bad valve - it oiled the plug. when i did the head several guides came out too easily.

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Can’t see that a whistle at the rear carb can be eliminated by a head overhaul, Tony - but concern about valve guides as indicated by rapid vacuum fluctuations is likely reason enough for a valve job anyway…

A bit odd after the ‘quality rebuild’ you mention - valve guides would certainly be inspected, though not changed unless deemed necessary. What is the actual vacuum readings - assuming also that the vacuum flutter disappears at increased rpms? The characteristics of the vacuum readings is indeed a valuable diagnostic tool…

Uneven idle is common enough on the xk - with a wide variety of causes…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
^^

when applying the ear & tube method, at idle, the air intake rear carb sounds turbulent, like something is spinning in a wind tunnel…as the RPM is increased to 1000-1500, the sound becomes a whistle…and that is the ~frequency of a human whistle.

The vacuum is in the 18-20Hg range and fluctuates very rapidly about 2hg

there appears to be an imbalance of air flow front to rear, and that may be due to a malfunction in one part, …concerned a valve guide may have worked its way loose

The head was “rebuilt” and has tappet hold downs fitted, but with guides, its possible for an incorrect interference fit to be an issue

the (apparent) imbalance in airflow (seems to me) the cause of imperfect running, and cannot be corrected, so I will need to pull the head and examine all the parts especially the 3 rear cylinders to see if I can find any fault, unless I can find some other explanation, and correct it, which would be greatly preferred

Have you performed a leakdown test?