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MG MGB Technical - 25D and 45D

Hi All

My friend has a 1968 GT, he has just had trouble getting the dizzy out and had to muller it to retrieve it.
It was a 25D with vacuum advance with thumb screw.
On ordering a new one they have send a 45D, my question is is this going to be OK for his car.????

GJ Barker

It depends whether the disi in question has the right advance curve.

They have been building 25D replacements in 45D bodies for some time since the 25D bodies became unavailable some years back.
Chris at Octarine Services

Can you explain the difference with the advance curve please between the 25D and the 45D.

GJ Barker

Not easily or helpfully it is a complex topic!

There should be a number stamped on the side - the original 25D should be 40897 or possibly 41288.

The 45D replacement may have either one of the above numbers, 101BY1 or 4127 stamped on the side.

45D distributors for the later rubber bumper cars will have 41610 stamped on the side.
Chris at Octarine Services

AS Chris says it is complex, but might this help? I had the same dilemma.

The D45 will run the car OK. Whether the curve is as "good" as before you won't know. But whether the original curve with this engine, today's fuel etc, is still the best, is also something you wont know. There is doubt as to whether modern copies have the same curve as the originals anyway despite what may/not be stamped on the body.

So, fit it and then adjust it till it is "just before pinking"-you will spend many happy hours till you have it "right" = best. Then if you still have the enthusiasm....

Use a dial back strobe to give you the advance at each say 500 rpm point (fun using with your head under the bonnet at 5k rpm). That's the curve of that distributor which you set with the static advance of X*. Nobody can tell definitively if a tweak here or there would make a difference, tho experience would show if it was way out.

Thereafter the only way to adjust it is by changing the internal advance springs (tedious and only good for experienced folk) or what I did-fit programmable ignition,use a laptop and back on the road again for trial and error. This time it takes 30 seconds to adjust from inside the car.
Michael Beswick

personally I'd go for a good completely electronic dissy if I had to replace

that's what I did on my current midget and I didn't have to as I'd already had an Aldon igniter head fitted but I didn't know how good the rest of the mechanical dissy was so as soon as they became available for my model I fitted a 123 (non-programmable) and am very glad I did, noticeable improvements throughout the rev range

Peter Burgess may still have stock of 123 for the MGB

I also took my car in with the 123 fitted for an excellent value and performance tune-up and I'm even more glad I did that, even more improvements

personally I think a rolling road tune up is best done on a car that is fairly recently fully serviced to get the optimum of of it

Peter Burgess advertises on here and is also author of the re-released book 'How to power tune MGB 4-cylinder engines' and a very decent guy and trader -
Nigel Atkins

Graham, I have a surplus 123 for MGB, with vacuum advance, if you are interested in going down that road. It gives you various advance curves to play with. I no longer have an 1800cc.
As said above, whatever dizzy you have you won't get the mechanical advance it gave "out of the box". Truth is it was probably not that accurate even then and was always a compromise. The max advance is easily altered, how it gets there (the curve) is a product of engine speed the mass of the bob weights, the strength of the control springs and friction in the shaft, again not rocket science to alter. But it is guess work unless you simulate on a bench.
Allan Reeling

Thanks Allan for the offer but I have just committed to an Accuspark system.
While on the subject of Dizzy's what is different about the 123 and I noticed in the parts book that there is one with vacuum and one without so how does the 123 work.????

GJ Barker

Rather depends on which Accuspark system you get. Stealth is programmable to allow you to adjust the curve. An electronic trigger conversion(in place of points) is exactly that. You will get the curve that "it has been decided" is right. It is contained in the dizzy itself. That curve is no more right (or wrong) than your original.

A 123 has a number of pre-programmed curves from which you choose. A 123 programmable allows you to program it using a laptop. With /without vacuum in mechanical terms physically moves the base plate to change the advance. This can be replicated electronically. It's benefit is improved economy.

FWIW mine is an Aldon Amethyst, which uses the dizzy but has the advance springs physically locked out. The curve is then introduced by software (included) on a laptop.
Michael Beswick

The vacuum advance is there to give the sparse mixture, evident when the throttle is almost closed, a longer time to burn. It works off manifold/carb vacuum, which is usually high on small throttle openings, BUT with sports cams with valve overlap, there is so little manifold depression, that it is often dispensed with. Also track cars spend so little time on part throttle and economy is not an issue vacuum advance becomes redundant.
Allan Reeling

As an old fuddy duddy I don't possess a lap top, so it will definitely be a simple Accuspark system.

But thanks for telling me about these modern devises.

I grew up with Austin 7's so I like to keep it simple.

My modern car has all the trickery on it and I can't play with that, if that goes wrong it's in a garage.

GJ Barker

the standard 123 dissy is pre-programmed so no laptops involved (I don't have a laptop and don't like them)

as I put earlier I went from an Aldon igniter head fitted in existing dissy to a fully electronic standard 123 dissy

the Adldon igniter head was good but the standard 123 fully electronic dissy was much better
Nigel Atkins

Vacuum advance allows for more advance on a light throttle, less advance on a heavy throttle. With a light throttle there is less chance of pinking, and the greater advance means that more of the burn occurs with the piston higher in the bore, which means more of the energy goes to forward motion and less as waste heat. This give better cruising economy than with no vacuum advance. Open the throttle, the greater charge in the cylinder results in higher cylinder pressures, which increases the chance of pinking, so the advance in reduced to compensate.

As said, that is irrelevant in competition, and the 2-dimensional mapping of non-vacuum distributors (rpm vs advance) is much easier to tune for ideal advance through the rev range than the 3-dimensional of a vacuum distributor). Unfortunately many assume that buying a 'performance' distributor i.e. one without vacuum advance is going to make their car go faster. It might, if it has a 'better' curve for the specific engine than the original, and/or the original is knackered, but that's about all.

Doesn't matter whether the take-off point is from the carb or the inlet manifold, the only difference between the two is at idle. Very soon after starting to open the throttle the two converge, and remain the same for the rest of throttle opening.

The biggest failing of the 123 is that it reproduces (or claims to) the original curves. Not only are the original curves irrelevant for today's fuels, but the original curves were the best that could be obtained using a relatively crude mechanical system that is really only capable of producing a steep advance to start with, then a flatter one. Far more accurate curves could have been produced with the technology, which is what a couple of the much cheaper options do. It's like designing a brand-new engine using all the latest modern techniques and materials, but restricting it to 110 ft/lb, 95 bhp, and 27 mpg.
Paul Hunt

the standard 123 has four curve settings in each group that you can switch to (and four groups of settings so 16 in total)

my Midget engine and it's connected input and output parts are mildly above standard, I had the timing set up using 95 octane as that's all I can usually get when going over my favourite Welsh mountain roads or the N.York/Cumbria passes but I also run it on 97/98/99 octane fuels for their cleaning properties and placebo effect

I also have twisty country roads runs in spirited fashion on the lovely and often empty country roads (if you know where to go) of Northamptonshire

my standard 123 was set on the first setting and didn't need to be moved up the scale

it runs very well on all the fuels on all the roads, my mate who tags along on many of the runs and Welsh trips follows behind in his 3 litre Toyota Supra and he's far from asleep at the wheel when following

my 123 has vacuum advance, one time for reasons I wont go into but nothing to do with the 123 I blocked off the vacuum advance and to be honest I didn't notice any difference but I've never done an mpg test since fitting the 123 as that would more discipline and less enjoyment

in the last 20+ years I've used various electronic igniters on various daily used classics including a Lucas system on my previous Spridget and Aldon on my current Midget and by far the best has been the 123 fully electronic dissy
Nigel Atkins

This thread was discussed between 05/02/2014 and 08/02/2014

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