More 123 Dizzy Questions

Please excuse me posting another 123 Dizzy topic, but after reading the many posts on the topic my questions remain unanswered. First, a bit of background. My 1969 Series 2 is completely stock with its original, but rebuilt, dual Strombergs. I recently installed a 123 distributor with the 16 presets. I made this selection because I do not feel qualified to map my own advance curves and I was queasy about the long-term support for the software-based versions. I selected advance curve #2 with static timing at 10 degrees BTDC based on Classic Jaguar’s suggestion and recommendations on this forum. I do not plan to install vacuum advance because the original Lucas distributor was purely mechanical. I reused the existing copper strand high tension leads with Champion “Bow Tie” connectors. The car runs very smoothly below 70 mph with moderate throttle. However, put your foot to the floor and try to accelerate above 70 and the car starts missing and has no power.

Here are my questions. (1) Do I have to replace the spark plug wires and terminals with something more modern. I love the clean look of the original setup with the conduit and Champion Bow Ties. If I have to replace the wires, can I still use the Bow Ties? I know many think they are crap, but I like them. (2) SNG’s web site says the Series 2 requires an external ballast for the 123. Is this correct? My car was not one that had this originally. (3) Advance Curve #2 reaches a maximum advance of 34 degrees at 3,000 RPM. Is this the optimal choice for a car without vacuum advance? The other choices max out at 36, 38, 40 and 42 degrees BTDC and at 3,000, 3,300, or 3,600 RPM. I believe the original Lucas mechanical dizzy had a max of about 28 degrees advance.

I should also confess that the high RPM stumble did not start with the 123. This suddenly came on last August and was not cured by a full tune up (spark plugs, points, condenser, cap). The Strombergs seem dialed in so the cause is a mystery, but I would at least like to get the 123 operating optimally before moving on. Thanks in advance for your input.

Bob the problem I’ve experenced with the bow tie caps is that they have an internal resistor, and that seems to fail relativly quickly. The symptons you experence, missing under power, is a classic failure of the system to deliver enough power to fire the plugs. While it might seem counter intuative the voltage requirment to fire the plug increases with load, and if there are problems with the wiring it just runs out of juice. The simplest way to check this is to determine the total resistance from the coil to each individual plug… Add in any resistance in the spark plug - if it has a R in the number it’s probably 5000 ohms. If the total of all this exceeds 15,000 ohms per wire that could be your problem. I say from the coil because resistance can come from the coil wire, and or a resistor in the cap electrode.

The original vacuumless distributors on the Ser II cars had a max advance of 37-41 degrees so that might be a better choice for a curve.

I can’t comment on the ballast resistor.

Assume your carbs have proper oil. Definetly replace plug wires. If that doesn’t fix then replace coil as well. The higher voltage with electronic ignition will find the weak spots in your old plug wire insulation.


From reading your post, I would say the ignition system is not your issue, because the problem predates the 123 installation. But you might want to look at your plug gap to make sure it’s in spec. Sometimes folks increase the gap with a new high zoot ignition thinking it will help, when it’s more likely to lead to misfire under high load conditions like flooring the throttle at 70. The 123 distributor does not increase the voltage to the plugs, only the coil can do that. Did you replace your coil, or are you using the stock unit?

To me the stumble seems more likely to be related to fuel supply, i.e. pump volume/pressure/level in the float bowls.

As to your specific questions: 1) No. 2) if your car didn’t have a ballast resistor and matching coil, then you are fine, as long as your coil meets 123 specs 3) optimal max advance choice is not dependent on having vacuum advance or not, but is dependent on engine specification. I would choose a max advance that matches your original engine spec. FWIW, I run 38 degrees total on my 3 carb 4.2 car with 123. Also, vacuum advance only comes into play during light load, light throttle running, so I would have chosen the vacuum unit for smoother light throttle running and better fuel economy.

I don’t believe the Bow Ties I use have an internal resistor, at least it doesn’t state they do. I know other Champion Bow Ties are marked as having a resistor. I thought one of my problems might be that I didn’t have enough resistance in the line for the 123. My understanding is that they need some resistance and I have copper strand wires. I do have R rated Champion plugs so there is some resistance. And yes, they are correctly gapped.

The stumble does predate the installation of the 123, but it changed after the installation. When I first test drove it, it fired right up and drove much better below 60 than it had before, but the stumble was still there at full throttle and over 70.

I didn’t realize the max advance was so high for the Lucas mechanical distributor. One joys of having a Series 2 if that the manual was written for the Series 1 and the Series 2 supplement is often less than complete.

In general I agree with you comment, but in this particular case I don’t. The supplement for the Series 2 does contain the advance curve for the distributors ( initially 41207 in your car, and later the 41322), but you have to know where to look. Offhand, I don’t remember where, but I know I’ve found it in the past. If you suspect inadequate fuelling it’s easy to measure the output of the pump by disconnecting the line at the engine bay filter and measuring the flow rate into a 2 liter soda bottle. If that turns out to be OK, I’d address the wires and plug caps. If you don’t run the 123 with the manufacturer-specified wires/caps there will always be doubt concerning whether that is the problem.


I’ve been looking for WCX600 caps without resistors for years. I like their looks and their ease of install and use, but let me assure you as of several years ago they don’t exist. I don’t mind the resistor in them, just don’t use a R code sparkplug, but IME the resistors don’t seem to last very long. As for yours - whether they have a resistor or are functioning is sooo very easy to find out…

This is what is in my supplement for the Series 2. Perhaps I am missing something (that has happened before), but this is for a Lucas 4106OA, which has a vacuum. My original distributor is a Lucas 41207A, which is purely mechanical. I did find some information elsewhere on the web which gave these three points for a 41207A curve: 2600 RPM, 17 degrees; 800 RPM, 7 degrees; and 400 RPM, 3 degrees. If this is added to the Supplement’s suggested 10 BTDC ignition timing, the max advance would be 27 degrees; however, to complicate things further, the badge on my plenum specifies 5 degrees BTDC for timing. So my confusion is not without basis.

I agree that manufacturers’ requirements for ancillary equipment should not be ignored, but the 123 comes with little more than cursory installation instructions. On this forum, I have seen people claim modern wires are required, while others report their twisted copper wires work just fine. Bottom line, the guidance is all over the place, which is why I am reaching out despite everything which has been written before.

The main body of the Series 2 Supplement addresses the non-US cars. The timing curve for the 41207 can be found in the special “Emissions” section of the Series 2 supplement to the manual (section Q) - this is where the Stromberg carbs are also described. The data points of the curve are listed on make QY.s.5 of the Bentley reprint. The advance quoted is 13-17 degrees at 1200rpm, 22-36 at 1600, 29-33 and 2900, and 37-41 and 4400rpm. Earlier in the same chapter it indicates static timing of 5 degrees and 10 degrees at 1000rpm. So, 6 data points in total.

I agree with you that the 123 installation instructions leave a lot to be desired. When it comes to the bluetooth configurable versions I’ve found more information on-line (from 123 themselves) than is provided with the product itself. I don’t know whether the same is true of the 16-switch version. That said, when I checked the information I have from 123 (for any of the distributors) I can’t find any specification for the required resistance of the HT leads/plugs. I think Pertronix specify resistor wires, and maybe that’s how some of us (myself included) have come to the belief that 123 do as well. As Terry implied in his comment, either way, it would be worth measuring the resistance from the distributor end of each lead to the inside of the spark plug cap for all your leads to ensure that they are consistent if nothing else.

My 123 distributor came with a rotor that had over 5k Ohms resistance. I believe they did that so even if you used copper wires there would be enough resistance in the secondary circuit to suppress any interference.

In your “Ignition Distributor Test Data” chart, it says to “Mount the distributor in a centrifugal test rig”, which is not an engine. That means that when they say to spin it at 1800 RPMs that means the distributor shaft is spinning at that speed. In order for that to happen in an engine the engine would need to be spinning twice that fast, or 3600 RPMs. By the same token, if they were to indicate 10 degrees advance that would mean that the distributor itself would advance that much, but if it were mounted in an engine the crankshaft advance would be twice as much as that.

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Silly me. Why did I think it would be in the section titled Distributor? Thanks for directing me to the correct information. Based on that, it would seem one of the more aggressive curves with more advance might be optimal, unless I use the vacuum option. As a side note, I have two Jaguars, the Series 2 and a 140, and neither one has vacuum advance. In the Series 2 case, it was for the emissions; for the 140 it was part of the Sandcast H8 package.

MIke, that is how I read it too. The distributor is running at half the RPM as the crank.

This has me scratching my head. 41 degrees seems like a lot of advance. Except for the emissions, is there any reason to try to duplicate this? In other words, is there a performance reason for that much advance?

It’s not about performance as much as giving the flame front a chance to get across that rather large hemispherical combustion chamber when the revs get high. So it’s more about complete combustion.

Open chamber (hemi) heads need more advance than wedge or pent roof designs.

Hemi heads have large undesirable combustion chamber shape configurations as the piston goes up, plus a sparkplug on the side. In order to achieve maximum cylinder pressure from combustion, at approx 17 degrees AFTC (the sweet spot for power and economy), they have to fire the mixture earlier than other engines with smaller more compact chambers, hence most hemis are set to a max of 40 - 45 degrees btdc at higher RPM’s This ensures that the flame front from the burning gas advances to the far side and consumes all the gas. The boogy man is that hemi’s are more prone to detonation which is an octane thing, than are other configurations. In the years following WWII Jaguar set their timing specs to the worst gas their customers could see and needed really modest timing to avoid detonation, which can destroy an engine. I suspect by the 70’s good gas was the norm and it wasn’t necessary to have such low numbers of advance. Second as the engines lost power it’s possible Jaguar tried to pick some up by specificying what is normal advance for hemi engines running on decent gas. So yes you will get more power from that kind of advance etc but how often are you at that rpm?

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Bob I had the same problem with my 1970 series 2. Going to Seattle anything over 70 mph the car missed real bad. I backed it off to 65 and it was OK. I would change your sediment bowl filter and see how it runs. Mine was full of debris on the inside. Solved my problem.

Thanks for the tutorial on hemispherical combustion chambers. Since there is such a benefit to more advance at higher revs, I wonder why the advance curves for the vacuum assisted distributors hit their max much earlier and with substantially less advance? I also appreciate the tip about the sediment bowl. I have checked almost everything I can think of so new ideas are appreciated. The car does have a newish fuel pump. The old one gave me an unforgettable lesson on fuel starvation. As I recall, it felt like running out of gas (on a freeway bridge no less) rather than missing.

I imagine that the main motivation for the extra advance at high revs for the 1969-71 distributors (41207, 41322, 41323) was, as per your first quote, mostly to do with getting complete combustion. The driving force behind the changes at this time was emissions, and incomplete combustion would be a big no-no as it results in high levels of carbon monoxide and carbon particles.

Well it’s an rpm/flame front speed thing. As rpm increases the total time at and near TDC decreases, but as the flame front doesn’t really speed through the chamber any faster it has to start earlier to complete combustion. I know your next question - yes the combustion process with what was retarded timing with an earlier distributor was more inefficient and resulted in less hp and higher fuel consumption - and other nasty things like more engine heat and oil/gas dilution etc.

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Do you want it run great, or use horrible smog settings?