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MG MGB Technical - 123 ignition.

I,m thinking of buying a 123 ignition dizzy. Quite expensive. Can anybody recommend it. I have read a thread on them but cannot find it. Chris.


Check out the MGC BBS. There is a lot of information about this distributor there. It appears to get excellent reviews from those who have invested in it, although, as you say, quite expensive initially.

That said, I'm saving my pennies for one for my MGC.

R Lynex

I've got one on my MGA (with MGB engine) It is very good, but not cheap. Easy to set up with pre-programmed curves and built-in LED for ionitial static timing. The thing seems very well built.

Since my engine set-up is not standard (different carbs, camshaft, manifolds and head) I opted for the GB4 version of the 123ignition instead of the MG4 version. They have different set of curves and seemed to suit my needs better.

I am still experimenting with different curves - but already I found out that it is a great improvement over the original unit - so I can recommend it!


Huib Berger

That's the bit that I don't get about these, they already contain pre-set curves. These are without a doubt better and more suitable than the original curves, but as they aren't really set up for your specific engine, it seems a tad expensive to me...
Can anyone compare the 123 to e.g. an Aldon modified dizzy, both installed on the same modified engine?

Alexander M

The MGB version of the 123 contains the *original* curves. The problem is that the original curves are no longer that relevant with todays very different fuels - unleaded and much lower octane. It does allow you to switch between the 16 original curves, and you may find one that is better *with* today's fuels. However I think it is a shame you have to remove the distributor from the engine to be able to switch curves, instead of being able to do it just by removing the cap. I've also seen a cut-away demo model which shows the components and PCB under the 'points' plate open to the surrounding air i.e. not encapsulated. If it is like that on the production version I'd say there is a very high likelyhood of coolant from a leaking MGB heater valve getting inside and doing a lot more damage than it does to the points type. The plate may well be sealed to the body of the distributor around its outer edge but it does have that whacking great hole in the middle for the spindle, and there is a limit to how good a seal between that and the plate can be.
Paul Hunt 2

The 123 has the potential to be a geat distributor, and if I understand correctly, you can have a custom curve programmed into it for the best performance. Unfortunately, that just raises the price. If you want a stock curve which is what you are chosing from, why not have the original rebuilt for less than half the cost?
Jeff Schlemmer

Either Jeff is modest or just refraining from advertising on this board. He's the guy to do it.

Allen Bachelder

I've been looking at the Megajolt distributorless system. Just an idle thought now, but the system does look awfully inexpensive. The downside is that there's a lot of tweaking to do to get it running properly, but doing away with the old dizzy does have its attraction...
Adam Birnbaum


For MGB info on Megajolt Lite system, have a look at and click on EDIS (electronic distributorless ignition system) on the left hand side. Also see for a UK EDIS kit for the MGB. Still needs a willingness to experiment though as it is infinitely adjustable!

I D Cameron

I was thinking of a 123, but one of the main attractions of the MGB is the lack of electronics to go wrong with no warning. The only electronics on mine are the radio, alternator regulator and the rev counter, and that's the way I like it!

I settled for a good used 25D (and drilling a vacuum pick-up into the carb) to replace the 45D on my 77 RB.
I spent the rest of the money on blinging it up, with an alloy rocker cover, and a wooden steering wheel.

I'm not saying that there is anything wrong with a 123, just that I think

nice mechanical thing that I can see working = good
nasty electronic box = bad
Martin Layton

I have fitted the 123 ignition.Straight away the engine sounded 'sweeter'. I have'nt had time for a test drive as yet but it was very easy to install.The only adjustment was to turn up the tickover slightly. I'll let you know how she performs after a run. Chris.

Trouble is, so many people compare a new electronic aftermarket item with a knackered original and 'lo and behold' pronounce the new one so much better. On a standard setup simply fitting electronic ignition, even less so with a simple electronic trigger like pertronix, Magnetronic or Unilite, makes very little difference. And at least with Pertronix and Magnetronic which are fitted to standard distributors you can at least carry points and condenser as a 'spare'. With the 123 or Unilite if they go bad on the road you are stuffed. And aftermarket electronics are *not* as reliable as they should be, just read the disclaimers on the Unilite instructions, for example!
Paul Hunt 2

Paul is spot on about the danger of comparing a worn out old one wiht any new one.

I looked at the 123 and did not buy one. Adding electronics to mechanical distributor may have been the write solution in the 1980's but I don't think it is the way to go now. (I still have the aftermarket electronic ingnition unit that I bought in the early 80's in use)

In the 21st century I think for me the choice is go back to the original, in good condition, or go for a modern wasted spark solution, be it magajolt or another one.

Merry Christmas,

David Witham

The dizzy i removed was 3 months old. All new and a perfect dwell. I'm sorry you both disapprove.As i said i will let you know how it performs after a run. Chris.

Chris, If your MGB distributor was 3 months old and didn't run properly, I'd suspect the ignition curve was inaccurate. There are MANY replacement distributor that don't offer the proper ingition curve. Some are downright terrible. Yes, I do rebuild and recurve Lucas dizzys. A lot of people do. You would be truly amazed at what $30 spent in the right place will get you!!!
Jeff Schlemmer

I never implied that the 45D didnt run properly. I was thinking about purchasing a 123 and was offered one at a VERY good price, which i found to hard to decline. Thanks, Chris.

Hi David,

I've just fitted a wasted spark ignition on to my BGT and it is superb. The performance of the engine is transformed. It has taken a lot of effort, to design a new front pulley, with trigger wheel, fit brackets to the timing chain cover to mount the crank sensor, build an air-box with air temperature sensor, additional water temp sensors etc, not to mention the electrical work.

The biggest gain is not in the 'strength' of the spark, but the accurate timing and a 3D advance map compared to the distributor with just vac advance. Once mapped correctly the B series engine is very usable. Any advances like this will make our cars more practical as daily runners.

I've not tried a 123 ignition, but it looks well made, and should be more reliable than points, with less maintenance. Just new rotor-arm and cap.
MG Mike

There is not much wrong with Lucas distrbutors if mantained correctly.I set most mgb,s at 18-20 degrees at 1000rpm NO VACCUUM ADVANCE(disconnected)If your distributor is working right,you should have 32=36 deg advance at 3000rpm.Try it you will like it.the 123 is an over priced piece of CNC machining.
mick burke

well, there are all the curves in it and you can select the one that seems to match the year of your engine. BUT you can not buy the fuel anymore (in most countries) these curves were created for!

The price of the 123 dizzy is not very charming to your wallet and at 175 to 200 Euro you can get a perfect rebuilt Lucas with new counterwights and springs etc..

The Megajolt solution is not as expensive as the 123 and can be matched to your engine and style of driving and the fuel you can buy, but it also an electronic item that is as relyable as the 123, I think and a roadside service is impossible on bothe the systems.

Have your Lucas rebuilt or change for a rebuilt one and tune your engine as explaned in the workshop manual and you will allways be on the right side!

I tried different elecronic ignitions on my upgraded B and was most satisfied with the easy handling of my non vac original Sepecial Tuning dizzy that found it's was back to the engine of my car.


Unless highly tuned or modified I'd say *always* use vacuum advance, it improves cruising economy signficantly as well as sharpening part-throttle acceleration. Whether the vacuum is taken from the carb or the manifold is almost immaterial as off-idle they are pretty-much identical. If you have a choice go for carb, it probably puts less stress on the capsule and makes for easier starting. Not that an MGB should be difficult to start anyway.
Paul Hunt 2

Hi again,
I have now driven the B with the 123 installed, and set the 'correct' curve for my 18SV 847 engine. Definately runs smoother.I'm not saying its now 20mph faster than before or does 0-60 in 5 seconds less, but it is definately a smoother and more responsive running engine than before. I will probably try other curves when time permits but as Paul says to switch curves the dizzy has to be removed, which isnt hard but a bit of an bind if you wanted to try all 16. Having said that im very pleased with the result as it is so may not bother. My verdict, Exellent. Chris.

Chris, glad to hear its working well. Which curve did you chose?
Jeff Schlemmer

Hi Jeff,
I used setting 'A', as my engine is an 18SV 847. Do you recommend i try a different one? Thanks for your interest. Chris.

My next step would be to try the #7 setting, then the #2 advanced 3-5 degrees BTDC. Its really dififcult to say since the curves are not given, but those look like better performance choices.
Jeff Schlemmer

The curves available for the *MGB* version of the 123 are the original factory curves, see for a cross-reference from distrubutors to switch positions.

As to how the curves on those distributors compare with each other you might like to have a look at and click on 'Distributor Curves'.
Paul Hunt 2

Paul, the way the cuves are listed, they don't look stock. The totals don't add up properly on some of them!
Jeff Schlemmer

They often didn't in the original information either :o) In theory static advance plus additional centrifugal advance at the strobe setting rpm should equal the timing for the strobe setting. In the info for early distributors I have this is indeed the case. In some later distributors it seems to be somewhere between zero and the static setting, in others it has even less correlation.
Paul Hunt 2

I talked to the designer/builder of this distributor, and the issue I have is that he does not take into account cars that have a manifold vaccum setup, rather than a carb vaccum setup. He said that the manifold vaccum setup doesn't exist..

It is common knowledge that the the manifold Vaccum setup was a NA Emissions thing, and is not a good arrangement for getting the best performance from a B engine, but if you buy one of these things, and connect up the vaccum on your NA manifold B setup.. I cannot see how this will work properly for part throttle driveability etc...

Marty Schmitz-Hertzberg

It shouldn't matter. The only difference between carb vacuum and manifold vacuum is at idle - carb gives zero vacuum and additional advance whereas manifold gives near max. As soon as you start to go off idle carb rapidly rises to meet manifold, then over the rest of throttle movement they both gradually reduce. It was moved from carb to manifold as the latter results in less pollution at idle. The down side is that it can make starting a bit more difficult and possibly 'stress' the vacuum capsule diaphragm more causing earlier rupture. However if the designer of the 123 has done more than simply stick to the same relationship between vacuum and additional advance as on the original distibutors which (for the MGB at least) the switch positions are supposed to replicate, like do something clever/additional with the signals, then having maximum vacuum at idle from manifold vacuum *could* cause a problem. It's certainly going to need much more care and/or expense changing from manifolfd vacuum to carb than going the other way (should one want to).

UK spec cars changed to manifold vacuum for the 77 model year but kept the same engine and distributor.
Paul Hunt 2

I agree with Paul that the difference between carby port vacuum advance and manfold vac is at idle and both behave in a similar manner under nornal running.
Its a bit hard to tell from the information on the 123 sight wether the total vacuum advance also adjusts with the change when you select a different curve. If it does it would pay to select a curve with a not to large vac adv if using manfold vac. I did notice that the static (not running)advance is very low on all these curves and this could help take the load off the starter. Also I/m wondering what the gear change retard is about. Denis

Info received yesterday is that the response to vacuum *does* change with the different switch settings, in the way that different distributors over the years (which the is what the switch settings relate to) had different vacuum capsules. Opinion also was that it also takes into effect whether the source was carb or manifold. Not so sure about that last bit, according to the information I have the UK spec engine and distributor stayed the same throught rubber bumper production, even though the source changed from carb to manifold for the 77 model year.

'Gear change retard', or at least removal of *additional* vacuum advance during a gear change, is what carb vacuum always did - assuming one lifts off the throttle to make the change. Low advance at cranking makes starting easier, which is why manual advance engines many moons ago had a lever on the steering wheel with 'start' and 'run' settings. You could break your thumb when hand cranking if you left it in the 'run' position and it kicked back, if you didn't also have all fingers and thumb on the same side of the handle. I've always felt that this is one of the drawbacks of manifold advance as it delivers near maximum advance with a closed (or nearly closed) throttle which is how I suspect most of us start our cars. I've never actually measured how much vacuum is actually present at the manifold port during cranking feet off, must remember to do so.

I think some of the 'features' of the 123 are a bit overstated, it can only sense ignition voltage, engine revs and vacuum after all and make certain assumptions from it. Other than that is doesn't know whether you are starting, changing gear, or anything else.
Paul Hunt 2

What I find really interesting in all the threads about the 123-ignition is, that EVERYBODY who fits it, is positive about it and that EVERYBODY who disapproves never tried it in a car.
I still have to find an owner of an MG with a 123-ignition who says it did not deliver.

I donīt have the system fitted in my MGs (64 MGB and 69 CGT) but I have contactless ignition systems in both (good old Lumenition in the B and a Newtronic in the C.
And I will never forget the day we had the C hooked up to a BOSCH engine tester. The engine was completely overhauled, with new coil, new Doug Smith distributor, new spark plugs and new plug leads. With this setup we saw abot 10.000 Volts at idle at the spark plugs, which got down to 5000 Volts at 3000 rpm and higher.
Then we fitted the Newtronic and without any other change the voltage rose to over 15.000V at idle and - more importantly - at higher revs the sparks still received 10.000 Volts at revs up to 5500 rpm.
The car was transformed, was revving easier, was much more responsive and could be revved up to 6000 rpm. With the standard setup it just didnt go higher than 5500 because there simply was not enough spark for it.

Now to have this advantage coupled with an accurately defined and even adjustable advance curve like 123 offers must be a dream.

Emperor's New Clothes...?
Neil Lock

The 123 is the only currently available electronic system I *would* consider. But it's major drawback is one of its selling points, and that is it accurately replicates the original curves. That would be fine if we were still using 4-star leaded but we aren't and many of us find we get pinking at the manufacturers specs even on 98 and 99 octane. I've no real desire to spend what is a considerable amount of money on what is a touring-use car only to find exactly the same thing happens.

Since they are orange-lined at 5500 I wouldn't take mine any higher than that, in fact I don't even go about about 5000 as anything more just sounds so painful for the engine. And seeing as how the peak torque is at around 3000 there doesn't seem much point in going way above that anyway.
Paul Hunt 2

Have you checked the curves for the 4 cylinder GB version of this distributor to see if any would be more suitable for the current fuel available. This is the unit recommended for those of us with a supercharger on board. I am definitely not a curve expert and am not trying to suggest this would be the answer but hope those that are might pipe in.

One would need to put an example of each different engine on a rolling road. And I'm not even sure how useful that would be, given the wear and different components on different examples of the same engine type. It would also need to be done on typical high and low octane fuels, which may differ in actual octane (and certainly additives) in different parts of the world. All-in-all a pretty mammoth task!
Paul Hunt 2

Frank, the GB curve IS probably the closest to stock you will find to work well with a supercharger, but a custom curve would work much better. There have been enough dyno runs with superchargers to determine what curve to at least start with if you want to go the custom route.

I think the 123 will become a great distributor, once the ability to adjust it becomes available!
Jeff Schlemmer

The 123 *is* adjustable - between 16 pre-set curves. If you want a fully adjustable unit to construct your own curves you will need a different unit. The MGOC (for example) sells a Lumenition unit that allows you to control the centrifugal advance at 500rpm intervals for up to 13 points - at Ģ423! It also doesn't include vacuum advance and so is really only applicable for competition use. Even then you will still have to have a session on a rolling road to determine the ideal curve for your circumstances or spend a lot of time experimenting.
Paul Hunt 2

Do yourselves a favor and before you throw big bucks at a hope send your dizzy to Jeff for recurve. Jeff has done a load of them for many of us and everyone is blown away what a difference it makes. Best of all its original looking. A lot of people bag on these stock dizzy's because they never had one set up right with the proper curve.

I'll bet Jeff would do a money back guarantee. If he won't I will. His work and integrity is that good. He's one of the best things that happened to my car during the restoration.

But what curve do you ask for?
Paul Hunt 2

Like the emperor and his clothes, you choose a curve that makes you feel good. If you want a curve that suits your engine and the fuel you use then a rolling road is your only option as you say. And then you'll get people who say that is only suitable for setting the advance for maximum power - to which I'll respond (to save lengthening this thread!) that if you don't want maximum power then just retard it a bit more than necessary and it'll be fine. I'm not against this distributor, it may well be better than a fixed-curve part as you get the chance to experiment. I'm against the idea that someone can determine an advance curve without running the engine...
Neil Lock

I've run EDIS on all of my vehicles for years now and they are rock solid. As far as repair is concerned, that's a simple matter of replacing stock Ford parts you can get anywhere. I'm using the stock module, pick-up and coils which leaves only the wiring and advance control signal, which in my case comes from a Megasquirt controller on the MG and the truck. BUT, I have run them with NO advance input, meaning it'll get you home even without the timing controller. Pretty fail safe I'd say. The VR sensors rarely go bad (If there's a big enough EMP pulse to fry that you have bigger worries), the coils... well, a coil is a coil is a coil, you have some redundancy there. I have 4 coils (V8), surely it'll run on 3 or even 2, and if you're really worried, drop a spare module in the glove box and go. Takes about 30 seconds to swap. I leave the spares on the shelf. Now you tell me how a points distributor is better than that.

Looks to me like the kit for EDIS/MGB puts the module into the controller housing, perhaps with their own circuit. I wouldn't like that. I think going away from the stock module is a bad idea for the above reasons, as well as the fact that the stock unit is a very sophisticated and well proven piece and almost any substitute is going to be inferior in some way. My opinion, add the controller to the stock system but leave the module alone.


Jim Blackwood


you are perfect right! I had the edis im my Ford and run it for more than 300.000 Km (150.000 miles) until my son crashed the car. There was never ever any problem with the EDIS and I will have it installed to the next B-series engine that I am planing for my roadster.

There are so many super special and intelligent aftermarked systems offered and the major gain they to put your cash into the pockets of some merchands with fine marketing ideas.

I can only agree to what you said and to a system that has been well tested and is kept so simple that it will last for many years to come with easy available spares nearly anywhere you go.



This thread was discussed between 14/12/2006 and 25/01/2007

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