Very strange ignition problem - really need some help!

Yes David, you are correct the current configuration has never worked properly. When breaking in the motor I never took the revs up high enough for this issue to rear it’s ugly head.

Yes, I am using Pertronics (Fairly sure it is LU-166, I can’t find model on the unit it’s self) and yes the coil (Flame thrower) is 3.0 Ohms

Numbers on the dizzy (had to use two mirrors!) are:
41064B (98% sure)
47?74 (?=this appears to be a blank)

I did not know Pertronics did not like copper wires…

Any idea if I have the correct dizzy?
Thanks, Pat

That almost sounds like a fuel delivery volume/pressure issue.

I felt it sounded like a fuel problem also. Try to observe the fuel filter to see if the glass bowl remains full when running rough. Also, could that the old fuel pump isn’t up to the job of delivering to the 3 SUs.

I responded to your private mail too, but here is is again.

If it’s a 41064B, then it is a 1966 model intended to 3.4 and 3.8L Jaguar engines according to: https://mgaguru.com/mgtech/ignition/pdf/lucas_distributor_specs.pdf
It is from the 22D6 family. The “correct” distributor for the 4.2L with tripple SUs would be the 41060A. If you look in the table in the linked document, you will see that the “all-in” spec for mechanical advance on the 41064 is 13 distributor degrees (26 crankshaft) at 2000 distributor rpm (4000 crankshaft). The figures for the 41060 are 9.5 degrees at 2300 rpm. So, your distributor could be giving you 4.5 distributor degrees (9 crankshaft degrees) more advance at 4000 rpm than it should. This assumes that your 41064 hasn’t been modified with new springs to conform to the 41060 spec. If the engine isn’t under load, this may not cause a problem, but would certainly be an issue when driving.

I’m not familar particularly familiar with the Pertronix product, but if you search the archives you’ll find multiple references to advice to not use solid copper wires with Pertronix.

My advice would be to measure the actual advance your distributor is producing (disconnect and block the vacuum port first) and various revs with a timing light. If your tacho isn’t working, you will need to find something to give you an accurate measurement of the engine revs. Your symptoms could be due to the centrifugal advance weights being stuck (so advance doesn’t vary at all), or maybe due to excessive advance due to the wrong springs.

Good luck!

I suggest you try a different rotor in the distributor and in addition, check that the vernier adjustment for the vacuum advance is in the centre. A defective or slightly shorter rotor can cause this problem. In addition, if the vacuum advance is not roughly centered on the vernier, the rotor may not be right next to the electrode in the distributor cap and thus the spark may have to jump more distance

dennis 69 OTS

The fact that the engine improves as soon as you throttle back as good as proves you have insufficient fuel reaching your twin SUs. If it was a gross air leak in your modified manifold it would behave differently.

What, exactly and completely, have you done to rule out a fuel delivery problem (from tank sump to needle jet)? Have you run the car with the fuel cap loose?

Peter,

Do you know something that the rest of us don’t? I assumed the OP had tripple HD8s on a regular 4.2 manifold fitted to his Series 2.

I agree with Terry. When I have had any similar symptoms like what you describe, it ended up being the mixture. If you have another set of needles, you might want to substitute them and see if it changes the timing or degree of missing. Another possibility is that as your timing advances, it may advance far enough that it pushes the rotor past the point where the spark can jump to distributor contacts. I don’t know how you can test that theory though, except try retarding the idle/low RPM timing by about 5 degrees and see if that makes any difference.

  • I tossed the Strombergs and put on fully restored SU HD8’s (carbs are perfect, but heck you never know right). Due to the SU’s going on no secondary throttle plates are installed so it’s not them.

I took this to mean two SUs and no plates. I guess it’s ambiguous?

I see your point. We both read the same thing and jumped to different conclusions. Can the OP clarify please?

Update: I tried another set of plug wires and caps, no joy. I am currently trying the correct Dist as per above advice, my gut says this is the issue… time will tell!

To will try and clarify some questions

  • How can I be sure the fuel system is not the issue.
    As the car is just finished being restored, I had the gas tank cleaned and lined, new hoses/pipes to deliver the fuel, new fuel bowl that is always full, new fuel pump, and fully restored HD8’s by this company (luckily in town) who does SU’s for people all over the world https://www.sucarburetors.com/ , super high quality.

  • I have original intake and exhaust manifolds, got rid of the 2 strombers and went with 2 HD8’s as you know.I don’ know what needles are in the carbs, but I can say the correct ones for where I live :wink:

I have to correct my self, no miss in motor until I reach around (not sure as tac not working) 4000 and then it dies.

Have tired changing the mixture, no joy.

Heading out now to finish installing the “new” correct dist.
Pat

Somewhere I’ve seen a test procedure that spells out how much fuel the pump should put out in a specific period of time. Unfortunately, I can’t find it in either the Haynes or Bentley manual though. There are three filters that could be checked. The one at the end of the fuel pickup, one in the fuel pump and the one under the bonnet. partially blocked.

And the two in the carbs.

Normally it’s enough to check the output of the pump by disconnecting a fitting between the filter bowl and carbs. Ten miles per gallon at 120 miles an hour would would need a gallon every five minutes or a litre per minute (ball park). Anything around that would not starve a twin carb six. BUT, if everything else discussed has truly been ruled out, including low float settings etc, you have to check the weird stuff m. For example, the fuel passage between the float bowl and the jet holder.

In case it helps I recently bought a used 41060A to rebuild and then play around with in my '68 to see how I like it compared to the original distributor since I read here on other posts that the 41060 has the best advance curve for a 4.2. Below is photo to show the centrifugal advance mechanism and the degree stamping on the advance arm. You might be able to compare to what is in your distributor. FYI when I got this one it was frozen and took a lot of soaking to free up the centrifugal advance. Next step will be to get a new vacuum advance and adjuster wheel for it.

David
68 E-type FHC

It might be your SU carb needles. My car ran wonderful up to about 3200 rpm and then I would get a pronounced engine stutter. She didn’t want to rev any further without a lot of effort. We did testing including swapping out the distributor. It turned out the carbs were leaning out/not enough fuel at higher rpm’s. We changed from standard UM metering needles to UE needles. Now, she runs like a champ with no stuttering and she revs to 4000+ without any hesitation. Give it a shot.

My research indicates there is a 7% decrease in diameter of the UE needle and the UM needle (from 2.545 mm to 2.375) at full throttle. This decrease in diameter of the needle increases the richness of the fuel mixture as it allows for more fuel versus air. This 7% decrease in diameter increases the richness by 23% at full throttle. The increase over UM for other needles are UB 11%, UI 16%, UE 23% and UO is 37%. All needles have the same fuel mixture ratio at idle. Go from UM to UB to UE to UO.

Oh yeah, I forgot, SUs.

For some reason an unrestricted flow of a quart every 45 seconds comes to mind. I wish I could recall where I read that spec.

This may be a silly question but did the place that did your SU carbs understand they were going on a 2 carb manifold and not a normal 3 carb 4.2 E-type SU set up ? Seem like each carb would need to be set up quite differently than on a 3 carb E.

David
68 E-type FHC

1 Like

I’m sure that’s it now that we know it’s a twin carb non-standard setup, assuming the delivery is in the quart/45 sec and litre/60 sec ballpark.

Many years ago I converted my series 2 xj6 with twin strombergs to twin hd8’s. I found that it began to lean out and stumble at around 3000 rpm and would then run fine if I switched on the hisser. Checking the specs I found that there is a different carb spring (stronger). specified for the twin hd8 xj6’s but the needle spec was the same. Changing out the springs solved my problem immediately. Sorry, I cannot remember what spring is recommended but there is a chart available. You could try adding a heavy washer on top of each of your carb pistons (below the spring). to add a bit of weight and richen the mixture as a test.

Clearly #3 & #5 pistons have holes in them. Pull the head.
Just kidding - I’ll bet its wrong needles, way too lean.

I just don’t think an advance curve is causing this. Timing marks would have to be on the other side of the damper to bog/stop the engine. On a V-8, made in the E type’s era I had the plug wires one wire position off. (283 vs 327). It was ran like a pig. But it never fell on its face given throttle. What was that, 40 degs off? No, diz degrees….I dunno, used a lot of gas. It was killing me at 28 cents a gallon.