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MG MGB Technical - 123 re-ignited

Hi guys. I realise this has been discussed before, but i could not find my specific answer.

Firstly, i should say that my mechanical knowledge is limited, but i am very keen to learn and the MG certainly has given me lots of opportunities to learn more!

The 123 has 16 advance-curve settings. Great. But what does this mean though? Which one is the right setting for mine?

I have a '72 MGB GT but the engine is a recon. I think its a red-seal or blue seal. Not sure. How do i find out what type my engine is?

Taking into consideration modern fuels etc, which setting would users of the 123 reccomend for my engine?

I would really appreciate some help here, in simple terms please :)

R Ackerman

The 123 for the MGB has all the *original* curves programmed into it, the theory being that one simply selects the curve that was originally applicable to the engine. However that rather falls down with the very different fuels we have these days, particularly with the lower octane. With the original timing one can expect pinking on 95 octane unleaded compared to 99 octane 4-star leaded, and even on 97, 98 and 99 variants in my experience. The best you can do is set the timing to give the most advance short of pinking at any combination of throttle, revs and load. But if you normally drive on relatively flat terrain even that is likely to pink again if you go on a trip in a more hilly area. The benefit of the 123 is that at least you know what the curves are and that they will be stable, unlike mechanical advance where very few of our distributors are likely to be to spec any more, new springs aren't available, and rebuilds are to very variable quality, curve-wise. The other benefit of the 123 is that with 16 curves you have the option of trying different ones in case one gives better results than another. However it is complicated by some retailers of the 123 recommending the *NON* MGB distributor for the MGB saying that it has better curves! One annoyance is that the switch is only accessible with the distributor removed from the engine. There are also vacuum and non-vacuum 123 distributors, you should get the vacuum type and gain the benefit of cruising economy, if you are interested in that.

Originally a UK spec 1972 would have had an 18V 581/582/583 high-compression engine with a 25D 41288 distributor, so that is the curve you would select on the 123. But if it is a reconditioned engine (and these are normally Gold Seal or Silver Seal for factory supplied engines and painted that colour) it could be anything, you would have to look at the engine number prefix. These were different on Gold Seal engines from the originals, 48G 733 equating to the 18V 581/582/583 above. If your engine number is neither of these tell us what it is.
Paul Hunt 2

Hi Paul,

Stupid question, but where i find my engine type number if its a recon?


Rudi. There are no stupid questions. On a factory rebuild, or a good quality aftermarket rebuild, the tag on the right hand side of the block, just below the cylinder head, should give you a basic idea of what you have. If there is no tag, assume it is an aftermarket rebuild and could be anything. Much may have changed on these engines since they were originally installed some 27+ years ago and you have to try and figure out what you actually have. But, if it is a factory rebuild, it will have a tag and that will provide a number/letter series which can be decoded to see what it was when it left the factory. It may, now, still be the same as when originally built/rebuilt, or it may have changed over the years. A lot of guessing involved, but most of it can be made with some degree of accuracy.

Les Bengtson

Might I suggest that you can get your own dizzy rebuilt and recurved to match your cam and the rest of your engine's operating parameters by Advanced Distibutors, ? I recently had a spare 25D dizzy rebuilt and recurved by Jeff Schlemmer, the owner, and what a beautiful job he did!! Truly excellent workmanship and remarkably good performance once installed. A shameless plug, I suppose, but then I don't give them out too often.

Bob Muenchausen

I should have added this photo to the previous post. What you cannot see is that there is NO slop in the shaft! And the quality of the materials used for the replacement breaker plate assembly and the points is better than anything I have ever seen done by anyone else. Much better even than the brand new EuroSpec dizzy I bought some years ago, and with better performance from the engine to boot!

Bob Muenchausen

Hi thanks for that. I've actually already purchased the 123, just need to find out what the setting is. I suppose the first step is to find out what my engine type is and work from there.

I'll try and find it tonight (hopefully!)

Thanks for the help.

One more photo for those who might be interested in what Jeff's work is like. If you go to his site, check out his areas on how he will rebuild your dizzy and the testimonials of the many folks who have used his services. My own criteria has always been to look for a consensus of positive opinions, and not only does he get it in these testimonials, but on other BBS's whose members have used his services. Also, take a look into his upcoming products ~ he is working on a 123 type dizzy which may be well worth a looksee. FWIW

Bob Muenchausen

Rudy, pick a curve that offers around 10 degrees of distributor advance in by 3000 rpms. If you bought the universal version of the 123, the MGA curve is the best one. If you can exchange yours for the universal, it does have the best curve. Otherwise use the early B curve from the 40897 distributor.
Jeff Schlemmer

Hi Jeff,

I have the universal 123. Which is the best curve, ie the MGA one? My instructions didnt offer model specific curves.


Hey guys,

Found this on Youtube, quite interesting:


To follow up on Bob's comments check out the Bargain Alerts, and specifically the thread “Rebuilt distributors for charity”

Jeff is donating proceeds to a friend with cancer to help cover medical costs. You can get both a quality product and benefit a worthy cause.

72 BGT
Larry Hallanger

Interesting (Youtube). Interesting to note almost the same amount of jitter on the pertronix as points, although I would have liked to have seen them both on the same distributor to eliminate wear differences between the two, and of course both those distributors were old as opposed to the new 123. However the 123 seemed to have its own significant but irregular jitter until it got into the higher rpms. On my V8 replacing the timing chain and gears (not a factor in this distributor machine of course), which I did as part of a top-end rebuild had the side-effect of removing virtually all the timing jitter, and that on a distributor with at least 100k and probably 200k.
Paul Hunt 2

I guess I'd try curve 2 with variations on the distributor position. Maybe try it at 0-5 BTDC.

I've commented on that before too. I don't like the way the 123 seems to tick like the second hand of a clock through the advance. I've also offered one of my rebuilt distributors for any to offer a fair comparison to their 123. No takers yet.
Any jitter in timing that's NOT due to play in the timing chain is likely attributed to a worn out shaft bushing, a sloppy breaker plate, or fatigued advance springs. The springs are the #1 issue. After 20 years they're typically junk. After 30 they're all but functional. After 40 years they barely hold themselves in place with very little function at all.
Jeff Schlemmer

Jeff can build you an engine specific curve for your stock bung or the SC. Merry, VEM
vem myers

In my experience the 123 is awful. Inconsistent advance which jumps about worse than a shot set of points in a worn dizzy!

In contrast an Aldon Ignitor (Petronix) fitted to a refurbed dizzy gives rock steady and consistent advance.
Chris at Octarine Services

Chris. I wonder how much is the difference in distributors? The Lucas distributors that I have tested evidenced relatively little timing scatter--no more so than any other quality distributor of similar vintage and condition. There is nothing wrong with the basic Lucas design and I am very found of the 25D series.

Like Jeff S., I have my own distributor test machine and have made up the tooling needed to rebuild a Lucas 25D properly. Unlike Jeff, I do not offer such services to the public, having done this as a research project rather than a business. Except as a favor to a few close friends, I do not do distributor rebuilds. So, when I say that my research data indicates something, I have nothing to gain by slanting the data in any way.

The properly rebuilt Lucas 25D distributor is more than adequate for most people's needs. Jeff and John Twist both seem to be capable of providing rebuilds of excellent quality and there are no other rebuilds that I would recommend.

I have, in my investigations, found two problems with the Lucas 25D distributor. (Beyond the problem of lack of factory original springs.) One is the straightness of the main shaft, the second is the grind of the points cam.

I have seen distributor shafts which were sufficiently off centerline to cause a run out of about .035". I have seen points cams which had up to .005" in variation when placed between centers in the lathe. (This is a method of determining how far the various lobes differ between each other. The net effect is different timing between cylinders. We set the timing using number one cylinder and assume that it will be, exactly, the same on the other four/six cylinders. Not true if there is significant difference in the points cam. In such cases, each cylinder has a different "ignition timing" and only the number one cylinder is correct.)

A Lucas distributor fitted with a straight main shaft and a points cam having no more than .001" of variation between the lobes makes a very fine distributor. Regardless of the coil triggering system used--i.e. points or "electronic" device.

The Petronix, and the variations on this theme, will compensate for a badly ground points cam, since the points have been removed from the system. It will not make up for a bent distributor shaft, nor will it fully make up for a worn distributor bushing. Thus, as I have pointed out many times before, the basis for any system has to be a properly rebuilt distributor if we are to make any adequate comparisons. And, proper annual maintenance of any distributor is still a requirement regardless of the form of triggering device used for the coil.

If one does not perform regular distributor maintenance, one should not expect the distributor to perform to expectations over any length of service.

Les Bengtson


some years ago i also tried to buy springs for the Lucas 25D. According to the MG manuals, they are NLA but if you have a look at the parts list for the TR3 with 25D, you can by a set of 8 or 10 springs that will work on our cars.




Ignition scatter is not just a function of the dizzy - it comes from the inevitable play in the camshaft gear/pinion contact as well.

Points exert a varying resistance to the cam lobe which at low revs cause the pinion to lose contact with the driving side of the camgear teeth and thus you get a jerky drive to the dizzy.

The points resistance varies from contact to contact and the timing is also affected by this.

The magnetic triggering of the Aldon/Petronix is not affected by the condition of the dizzy cam or the "wobble" of the shaft if it is minor. As there is no varying friction on the cam the dizzy pinion stays constantly in contact with the camshaft drive gear.

Smooth advance curves rely on the weights, springs etc being in good condition and properly lubricated - as long as this is so, the mechanical dizzy has a smoother curve than the step curve of the 123.

I won't fit 123s now - and don't recommend them. IMHO they are an expensive bit of "bling" that actually work less efficiently than a good Lucas 25 / 45.

Chris at Octarine Services

A bit off topic, but my rebuilt 25D arrived today courtesy of Mr Schlemmer, and fitted with a pertronix and Bosch wire set.

1 hour later, and she starts up first pop after simple drop in for the old dizzy and using out of the box plug gaps (0.035", I believe), and of course I couldn't resist indexing the plugs too :)

A lovely bit of workmanship - thanks Jeff! Even without fine tweaking the new timing and adjusting the mix, the seat-of-the-pants meter feels good!!
Curtis Walker

This thread was discussed between 10/12/2007 and 14/12/2007

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