Total advance on XK engine

One thing I don’t think I saw mentioned is that if you are directly measuring the angular displacement of the rotor, keep in mind that the distributor rotates only once for every two rotations of the crankshaft, so 5 degrees measured in the distributor is 10 degrees at the crankshaft.

Hi Ron. I had British Vacuum in New Hampshire rebuild and change the specs of my distributor when I converted my S1 XJ6 to triple SU’s (among other mods). He definitely sells vacuum capsules and other parts as well. No affiliation but he did a very nice job.

That looks like a good resource. I have used Advanced Distributors for a Lucas BMC 1275 distributor for my Morris truck, with good results.

Nobody beside Distributor Doc seems to sell springs, which may be because it’s too much of a compromise compared to a rebuild/recurve. When I get the time, I’ll think I’ll try to snug my springs up, then get a rebuild this winter.

So, progress report here.

I pulled the distributor and examined the initial advance spring, which to me looked stretched and weak.


notice how the last coil looks stretched. So, I snugged up the ends and tightened it.

Upon reinstalling, I noticed a prolific leak from my front carburetor, which resulted in 6 hours of work to fix, plus getting some nastiness out of the float bowls and changing the filters. I’m afraid my tank cleanout wasn’t fully successful, but the fuel is now clean at my see-through in-line filter. Anyone who has removed and reinstalled a front carburetor on a Mk10 knows why it took so long - b^& of a job.

So, I static timed at 10 degrees and started the car. Idle speed dropped about 75rpm from before the distributor work and crank timing decreased from 15 to 12 at 550 idle, which was smoother, not perfect. Went for a ride. New hard miss on acceleration, not improved by switching in the ASC, so probably not a lean miss. I verified vacuum assist working. Today, I gathered data for an advance curve, also scaled the ported vacuum.

Compared to the curves posted by Davidxk ,mine is flatter in the 1000-200 range, but I’m not sure it’s enough to cause a miss. Spring might be too tight. Might be something else going on. Also noted some sooty plugs, which has been an ongoing issue with this car - the air filter arrangement on the Mk10 is a real choke point, I’m learning. Tune the car with the air filter off, then have to lean it out when you put it on.

Also, the distributor cam was stamped “9”, which should give me a total advance of 28 degrees, but I obviously have a lot more, off the scale at 3000rpms. If I read the stamping correctly, this is a 1971 distributor in a 66 car. The vacuum assist bumps the advance up through the whole curve, but may not have as much effect if there was load.

All this is interesting, but doesn’t lead me to any strong conclusions other than getting the distributor recurved when I can. Plan is to replace the plugs with some NGKs, get my air/fuel meter working (new gauge ordered) and see about getting my hands on some high quality new points, condenser, rotor and cap. Maybe prayer.

Any thoughts?

I have a document that purports to give advance specs for a wide range of Lucas Distributors - I imagine that I originally found it from someone posting it here on Jag-Lovers, but I don’t know for sure. Here are some things I found from reviewing it:

  1. Your distributor 41065A is correct for a 4.2 Mark X with automatic transmission. If the car had a manual transmission, it should have the 41060A model, which is the same as the 4.2 Series 1 E-Types.

  2. The Spec for the 41060A (E-Type) in the document matches the curve I included in my earlier post. So, that gives some credibility to the document.

  3. The Spec for the 40165A includes the following break data point assuming 10 degrees static advance - all figures quoted in crank degrees and rpm (though the document itself is in distributor degrees/rpm) and exclude vacuum advance:

  • No advance from static level up until 600rpm (41060A E-Type 600rpm)

  • Total advance at 1300rpm is 16 degrees (41060A E-Type 1050rpm is 13 degrees)

  • Total advance at 2500rpm is 28 degrees (41060A E-Type 1600rpm is 24 degrees)

  • Total advance at 4600rpm is 29 degrees (41060A E-Type 4600rpm is 29 degrees)

  1. It’s worth noting that the first two data points in the 41060A E-Type list correspond closely with the inflection points in the 41060A curve with the third data point being in the “max’ed out” section. This suggests that the 41065A data points may be similar, which would indicate that (a) the initial lower slope section of the curve is longer (700rpm long vs 450rpm), and (b) the following steep section of the curve is longer (1200rpm long vs 550rpm) and much less steep (climbs 12 degrees in 1200rpm vs 11 degrees in only 550rpm)

  2. If we compare your measured curve with the above data it appears that:

  • Your advance is too low at 1300rpm (13 degrees vs 16)

  • Your advance is a little high at 2500rpm (31 degrees vs 28)

  • Your advance curve continues to rise beyond the point where it should max out

This would suggest to me that the initial spring is too stiff, the second spring is too weak, and that the stop pins that the prevent the advance beyond a certain point are worn, and allowing too much at the high end.

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You spent some time on this. Many thanks for that, and it’s most helpful. I suspected the problems with the springs you describe, but didn’t know what to make about the high total advance, so it’s good to know about the stop pins. I should to see if I can schedule in a rebuild. Hate to have the car off the road all summer, but it’s not usable as is. Or consider replacing the distributor. I might pull it out and have a another look at it. Look at those pins to see if I can tell why it’s over-travelling.

A rebuild or replacement may well be the answer. One thing that surprised me was the difference between the curves for the 41065A which is used for the automatic transmission model and the 41060A which is used for the manual transmission model of the same car. It wouldn’t have occurred to me to do a dizzy swap if I converted an auto to manual, but for the 4.2 Mark X it looks like it would make a huge difference. Hopefully someone else here can validate the accuracy of my above statements as it was a genuine surprise to me how big the difference appears to be.

Given the fact that an automatic engine operates in a generally much lower and more consistent rpm range, that makes sense to me.

Interesting.
The XJ6 may or may not have had different advance weights or springs, not sure right now, for manual/auto. I think it lists different part numbers in the workshop manual, but the lucas catalogue shows that only the rotor arm (speed limiter) ist different.

Maybe the automatic doesn’t load the engine as much in part throttle situations with the generally higher cruise rpm and the slip in the torque converter, but it is less lively and thirstier so they figured they could add a bit more advance safely.

I did get into this a bit today. I scrounged around and came up with a pretty nasty 22D, part number 40980E 1064. I don’t know what this came from and I can’t find a listing for the number, but I’m assuming '64 manufacture year. Is there a chart online somewhere? I can only find pre-1960.

The vernier is frozen, and the cams needed cleaning up, but the springs looked good, the posts looked straight, and the vacuum unit worked, so I moved my good base plate over, static timed the car at 10 degrees, and ran the car.

Here’s the new advance curve, which I did with vacuum only.


As you can see, much better advance in the 1000-2000 range.

On test drive, my acceleration miss is gone, so that was due to improper advance. I think the car is a bit sluggish, but it’s big! The new 700R4 transmission helps with that. I have a replacement air/fuel meter coming, as well as some new spark plugs, but I’ll be interested to see if my sooty plug problem changes. If it was due to poor timing, I may be able to go back to richer needles. Learning a lot here.

Of note is the persistence of high advance with the second distributor. I don’t understand it, but it might not be there if the engine was under load, due to lesser vacuum. Neither distributor cam looked altered, both stamped “9”.

So, I have a working distributor, and plan to send the other off for a proper rebuild.

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From the Lucas database.

41065A

22D6 CCW

2300 rpm 9.5 degrees
1250 rpm 9 degrees
650 rpm 3 degrees
300 rpm no advance

Vacuum 7/14/8 (y) push

1966-74 Jaguar/Daimler 4.2 MK X Auto, Sovereign 4.2 auto

So you should top out at 29 degrees at 2300 rpm.

That it’s advancing to 40 is wrong and might well be the reason you’ve got that miss, all else being equal.

Did you take your numbers with the vacuum detached? If not disconnect vacuum and check again. Your unit if functioning should start at 7mmHg, stop at 14mmHg and provide an additional 8 degrees of advance.

Thanks for your input.
As I mentioned, the miss is gone with the second distributor.
I did get my A/F meter going again, and re-tuned the car with good data. Changed the UM needles to UE and am now getting good performance, nice looking plugs. Turns out, by manual tuning, I was too rich at idle (although it didn’t behave that way with piston lift) to get reasonable, but still somewhat lean operation at wot. I believe improper timing due to the distributor malfunction may have had something to do with that, but it’s a bit mysterious. I’m a definite fan of the oxygen sensor approach.
Yes, I realize I should have had the vacuum disconnected, so I believe that was the error for the total advance number on both distributors. I didn’t think it would affect total advance, but, absent load, it apparently does. I’ve been told that vacuum doesn’t affect total advance, but that must just be true on the road. I’ll recheck it with the vacuum disconnected when I get a chance, but the car is running very well now.
Thanks for the application on the 41065A. Do you have a listing for the second one, 40980E? That’s the one I’m running until I get the first one recurved.

Good to hear you have got it sorted.

I agree with you 100%. I use a CO meter to tune my cars as I find manual tuning can be hit and miss.

I have an O2 sensor but as I have 4 cars with XK engines, plus a V12, I haven’t been able to decide which car gets it :slight_smile:

I think that if you are just revving the car in neutral without load you can get interference with vacuum, and it is unpredictable. When I graph a dizzy on the car with a dial back light I disconnect the vacuum, just to be certain it has no influence.

I’m getting the dizzy data from this excellent database.

It offers:

40980A
22D6 CCW
2300 9.5
1250 6
650 1.5
300
7/14/8

No car identified.

Hmm, interesting. There is significantly less advance at 650 and 1250 rpms than on the original Mk10 distributor (41065A), which also appears pretty similar to the 41060.

I’m going to package it up and send to Advanced Distributors this week, see what he recommends doing with it. While the car runs pretty well now, this data would suggest that it has too low of an early advance curve for this car. I do wonder how the presence of an overdrive automatic (700r4 including better initial band range than the replaced BW8) affects the distributor requirements. I would think the more modern transmission would behave more like a manual up to mid rpms, but of course, would never see the high rpms of the manual.
Many thanks. I’ll report back when the distributor expert weighs in.

Those figures @abowie quoted are for distributor rpm (so double them for crank rpm) and distributor centrifugal advance (so double them and add the static 10 degrees to get crank total advance). So they translate to no advance below 600rpm, 13 degrees at 1300rpm, 22 degrees at 2500rpm and all in 29 degrees (4600rpm, but it’s probably all in earlier - say 3500rpm). These numbers are lower than the 40165A but not dramatically so.

Ok, I’m confused by your comment. Yes I understand now about distributor degrees, but I don’t see how you get those numbers from the quotes for the 40980A
2300 9.5 10 + (2x9.5)=29
1250 6 10 + (2x6) =22 I’m showing 16 WITH vacuum, so clearly not correct for what I’m running.
650 1.5 10 + (2x1.5) =13 I’m showing 12 WITH vacuum, so more than mine.
Do you have a different source?

for the 41065A
2300 rpm 9.5 29 the same
1250 rpm 9 10 + (2x9)= 28 way too much
650 rpm 3 10 + (2x3)= 16 way too much
I didn’t generate a complete curve before I messed with the springs, but the early advance seems like what I saw with this distributor initially. I don’t think it’s the curve I want.

I would have to retest without the vacuum to get my real world numbers, but that’s nowhere close to what I am seeing, so I’m inclined to not accept those parameters as correct. It’s nice to have the chart, but do we know where the numbers came from?

But maybe I’m missing something here (again).

I don’t understand why you’re confused by my comment. Your calculations are exactly the same as I quoted in my post. All I can think is that you misunderstood when I said "

I was referring to a comparison to the SPEC for the 40165A, not the actual numbers you measured with your 40165A. Is that where the confusion came from?

Indeed. The measurements taken with the vacuum unit not plugged and an unloaded engine are not at all helpful.

If you are referring to the data points that myself and Andrew have provided, they come from a spreadsheet compiled by Marcel Chichak of TDC Engineering (the document Andrew provided a link to). I don’t know where he got the information from, but I imagine that it was directly or indirectly from Lucas. I do recall finding what appear to be errors in the past, perhaps when transcribing from one source to the spreadsheet.

David,
I am easily confused, so please don’t take it personally - I do appreciate your patience.

I was first referring to the specs from the spreadsheet - 28 degrees crankshaft advance at 1250 vs 22 at 1250 seems like a big difference to me, but maybe it’s not. I will try to get around to doing a no-vacuum curve on the 40980A this week. From what I know of this dizzy at this point, it’s specs don’t match the chart, but it’s old and of unknown origin.

My experience with curves on old Lucas distributors is that they are a lucky dip at best.

It’s a combination of general wear, spring fatigue and dirt inside the unit, all of which prevent it doing what it did from the factory.

You’re doing the right thing, which is getting your dizzy rebuilt to correct spec.

This article is well worth a read, and demystifies a lot about dizzy curves.

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My distributor is off for reworking. Jeff at Advanced Distributors told me that Lucas had many different curves for the cars of the sixties and would have been better off sticking to what they did in the 40s.
Thanks for the reference.