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MG MGB Technical - 123 Electronic Ignition Distributor

I asked this question on another thread, but perhaps nobody saw it. Any experience or opinions?

Under "Hotlines" my June 2006 issue of "Enjoying MG" (MGOC) shows, on page 9, the "123 Electronic Ignition Distributor" with adjustable advance curves. Is this distributor the answer to our prayers? I see they have models with and without vacuum advances. Does the answer to that question depend on what cam one is using? What carb(s)?

Many thanks!

Allen Bachelder

It reputedly uses the original curves, which because they are derived electronically and not mechanically should be a lot more accurate and long-lasting. However! The biggest problem today is not so much that the original curves cannot be obtained (which they can by tuning what springs are available) but more that the fuels available today are so very different, even if the engine specs are identical, which they rarely are. Because of that we should be testing our engines on rolling roads to determine what the best curve is taking into account current fuel and current engine spec, and tuning springs to obtain that. But short of that the best you can do is to adjust the timing so that you don't get any pinking under any combination of engine load, throttle opening and revs. But even then if you change fuel grade, or visit a part of the world with a different topography, then you will either be losing performance or getting pinking!

Having said that the 123 distributor seems a far better option that retro-fitting an electronic trigger to a knackered distributor.
Paul Hunt 2

Looking at the specs, they give the basic numbers for total advance at x rpm but don't disclose or include the chart of the advance curve in their published info.

Their hi performance curve continues to add advance on up to 4000 rpm. Typically these engines take lots of initial advance and not much more as rpms increase. Without looking at a graph who knows?

One point, fuel changes can easily be accomodated, just as with the stock distributor, by rotating the distributor and changing the initial timing.

The other new product, a modified gm distributor, looks interesting in that they will program a curve for you. But what happens when you change things and want to recurve? Do they charge to redo the timing? How about a computer program that allows you to retime using your PC.

With a computer ignition you don't need a distributor at all, but the vacuum reading is usually not used in most computer systems. Fuel injection allows programming fuel input with spark advance. On the carb cars, throttle position sensors are the inferior substitute or a manifold pressure sensor.

Barry Parkinson

Having spoken to the chap in Canada who programmed the curves for them he tells me that they have the standard curves for the many distributors that were used. Whilst rotating the distributor can be used to initiate/prevent pinking it doesn't alter the curve. You will probably end up with an actual curve that only touches the ideal at one point, and is way too conservative everywhere else. The original curves were conservative anyway, having to take into account the tolerances of engine manufacture plus a safety factor. In many case one could run with more advance (on leaded petrol) without pinking and get the benefit of better performance *and* economy, and in other one may not be able to go higher than the book figure. I have experienced both on different engines over the last 30 years.
Paul Hunt 2

Barry, I purchased one of the D.U.I. GM based distributors after looking at the 123. My application is a '67 roadster running the Moss blower and I find that the overall improvement is quite noticable. The shaft tolerances keep the spark scatter to 1 degree versus the stock 5 degrees at 5,000 R.P.M's. They charge $20 to recurve the unit for you at a later date if it becomes necassary due to future mods. Ray

After years of attempting to get the misfire out of my 73B I went to a dual point Mallory and could never get her to run smooth in the higher rpm's. A week ago I installed the Igniter II and immediatly the car ran like new. Other problems in the, now removed, air pump cropped up and I'm waiting for parts prior to getting back on the road.

The Mallory comes with a set of 6 pair of advance springs (no vacumn) I now have the lightest installed, this should give an early advance. I am looking for more low end torque. Do your think I am on the right track or as you mention will the igniter compensate by itself?
Ken Knize

Allen, if I remember, that distributor uses a MAP sensor to make up for no vacuum advance module, so it does take into acouunt the manifold vacuum levels. As Paul states, if you are looking for a medrn performance curve you are better off looking elsewhere. You could buy 3 rebuilt and recurved units (from me?) for the price of the 123 distributor.

The D.U.I. unit is a great distributor, if you don't mind the fact that it is HUGE and looks nothing like the stock unit.

Any functional aftermarket ignition system will alleviate issues with shaft run-out and points bounce.

Ken, there's no way of knowing what your ignition curve is, so it would be very brave for someone to say that you have the appropriate springs installed. Do you know how many degrees of advance your distributor provides?
Jeff Schlemmer

Thanks Jeff, I'll have to look back at the literature to see. I recall that it did show different curves.
Ken Knize

The backyard way to do it, is to time it by pinging.

Run low octane gas so your car will ping under load. Set up the distributor as far advanced as it will go without pinging.

If you find it isn't pinging at low rpms, but is at high, then you need more low rpm advance. You will need to change the springs for a different advance curve.

Vacuum advance systems have their own set of adjustments. Some have stiff springs and only advance at quite hi vacuum levels (such as the late smog engines) Some have less advance but softer springs so that the advance only goes away with a decent throttle opening.

The prior time I built my engine, the compression was low enough with the 5 port alloy head that it wouldn't ping no matter what, and I had to try and use the seat of the pants technique, which isn't nearly as good.

In general with a modified engine, you will have low effective compression ratios, at low rpms and relatively higher than stock at hi rpm.

Basically you need lots more advance at idle and low rpms than stock, and not a whole lot more advance above 2000 to 2500 rpms. Vacuum advance is needed for a street car to run well. The early vacuum systems are better than the late for street driving with a modified engine.

If you mark your crank pulley damper with lines every 10 or 15 degrees you can use a strobe light and get a very good idea what your distributor is doing. Try it with and without the vacuum advance connected.

You would certainly be able to do it better with a rolling road - dynometer, but it is important to respect heritage. The only rolling roads readily available back in the '60's, were commonly refered to as hedgerows.


Barry Parkinson

There are vacuum and non vacuum versions of the 123, i.e. equivalent to 2/45D and 2/43D versions. I've never seen any 'official' indication that a non-vacuum unit was used on the MGB, although there are indications from other sources that 75 and later North American cars might have, possibly as a cheaper alternative to having the vacuum type but with the vacuum advance disabled if that was the only way of getting it through the emissions test. Whatever, the MGB centrifugal curves are the same as the originals, i.e. the non-vacuum version does not compensate for not having vacuum, it just uses the non-vacuum centrifugal curves.
Paul Hunt 2

Many thanks, all!

One thing for sure: based on this discussion I'm not about to run out and buy a 123.

Allen Bachelder

I have read with interest the comments made about the 123ignition distributor.
I have had one of the MGOC 123ignition with vac advance distributors in for the last 3 weeks and I can honestly say it has transformed my roadster. Since fitting the 123, the car now pulls very strongly throughout the rev range with instantaneous throttle response and noticeably improved performance and power so much so that it is like driving a different car, all by just changing the dizzy. It also idles very smoothly and starts instantaneously with very little choke. From my point of view getting the 123 dizzy was money well spent, easy to fit and probably one of the best purchases I have made for my MG (and I have spent a lot!).
N Argyrou

A proper spark curve and vacuum advance makes all the difference in the world. Those little springs and weights down underneath the points harbor more potential than a set of new carbs. Using an early model spark curve with a late model smog engine can make a big difference.

Before the mid'70's, the goal was performance. Once pollution control was mandated, lowering emissions became the goal of the engineer. Changing the spark curve was an inexpensive way to reach those goals.

Once you modify the engine, the spark curve that was appropriate probably won't be.

The problem I see is that the 123 modified spark advance curve might be good for certain modifications and not for others. They are offering a one size fits all package for modified engines - and no way to alter it.

Barry Parkinson

If you want full control of the advance curve and mappable ignition, why not look at EDIS ? You should be able to fit a Megajolt kit for about half the cost of the 123 dizzy...
Andy Torode

After a day out, it was nice to come home and discover this discussion is not dead. There was a time when I thought all 25D4s were the same, but no more. Nick, it is interesting to hear some very positive remarks about this distributor, just so set the matter in some perspective. I'm going to try a properly rebuilt/recurved distributor 25D4 first. If that doesn't do the job, I'm going to have to learn some more about my engine.

Thanks again,
Allen Bachelder

fuel pumpe years ago I fitted an Aldon Distributor with an
MG Mike

Here's a copy of the email I rcvd from DUI re the HEI distributor for the MGB:

Hi Barry,

Due to design limitations, we cannot build the MG D.U.I. with a vacuum advance.

We charge $45 plus freight to re-curve one of our distributors; however, your motor change may not require recurving. Sometimes, you can run more or less initial base timing to accommodate the change. When you do make your motor changes, call us with your individual ID#(which we assign to every distributor we build), we will pull the file on it, and discuss with you whether or not you need to send the distributor to us t be recurved.

Performance Distributors
2699 Barris Dr.
Memphis, TN 38132
Barry Parkinson

Design limitations? They must be referring to the fact that the vacuum advance mechanism of an HEI would be a vacuum retard on a B. They would need a module with a rear-facing port to actually advance the timing.
Jeff Schlemmer


I e-mailed Performance distributors to see what they can do for a supercharger curve. I pretty much had the same answer as Barry. Well, more like it can't be done.

Ray 1977mgb

This thread was discussed between 09/06/2006 and 15/06/2006

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