MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Distributor identification

Hi,

I just wanted to confirm distributor on my 948 engine is correct. It is stamped 23D4. The engine is apparently from something else according to previous owner but still 948cc. Before I order spares I wanted to double-check. There is no vacuum advance on it.

If not the original spec is there any good reason this would be fitted or is it just someone using what was lying around?

Also, there's an ignition module does anyone know what it is and whether it is one of the good ones?

Thanks





P Peters

What does it say on the gold label on the side of the module. I can make out "....spark" so I presume it is an Accuspark. I've had Accuspark and had no trouble at all.
The 23D4 is not standard for the Frogeye, I would expect a 25D4 with vacuum advance. I believe the 23D4 was used on early 1275 midget engines.
There should be a 4 digit date stamp somewhere on the distributor. I think it should indicate Week Week Year Year.
Have a look here

https://www.teglerizer.com/mgstuff/advance_curves.htm
MG Moneypit

Yep, looks like Accuspark.

Dizzy model number is the 4..... number. 40819? I can't quite read it clearly on that picture. But that would tie up with being correct. Search for "Lucas 40819" comes up with a whole load of links/descriptions.

"Lucas distributors are identified by the 5 figure number stamped on the body, in this case 40819. Fitted to the Austin Healey Sprite/MG Midget in 1967 and Mini Cooper S from 1965 to 1969."

https://www.holden.co.uk/p/distributor_mini_cooper_s_a_h_sprite_midget_1967

Malc.
Malcolm

The original 948 has a DM2 distributor with a vacuum advance. The distributors are different and it may not have been possible to install the Accuspark on the DM2 hence the change. The advance curves are quite different between the two distributors and you may be losing some torque low down and using a bit more fuel unless the internals of the 23D4 have been changed.
Conversely has your 948 been modified? It will be fine if you have a better cam, flowed head, bigger carbs etc.

Of course your orginal thought of it 'lying about' may be the real reason!!
Bob Beaumont

Thanks all.

Yes 40819 K. The gold label is not readable - to me at least.

The car has been modified and unmodified over the years - the rear arches were rounded at one point but were back to normal when I got it.

I don't think the engine is modified as it doesn't like to go over 4500 revs though it will but reluctantly. Conversely it's quite torquey low down. So I assume standard.

It works well enough for now. Just planning spares for a longish trip. I'll check out the cost of the Accuspark for the 23D4 but also if there is a sealed take off for vacuum somewhere.

Just checked the cost of 25D4s and they seem reasonable on Ebay so might be worth a punt to see it improves the engine.

P Peters

Before you buy consider a word with A.C.Dodd on FB. There is a modern 25D that he rates because it has 59D internals. It may, or may not be the Accuspark item.
Alan Anstead

Regardless of what the stamped specification is on any old distributor, it cannot be relied on to be correct unless you can document that the specification has not been modified or changed over the years. The various Lucas distributors were set up in a number of ways, depending on the specific requirements of an engine, and a "specification number" assigned to all of the distributors built to that requirement. But, I have seen any number of Lucas distributors that have been either modified or rebuilt using different parts, resulting in a distributor that may not perform to the original specification. And, while you can (with a detailed inspection) determine some of the possible changes that may have been made, other changes (or simple wear) may not show up with a simple visual inspection. Remember, within a Lucas model (eg the Lucas 25D4), all of the internal parts will interchange.

As a result of this, you have to do an actual test of the distributor to know what you actually have and then you have to compare what you actually have with what is the required specification as listed for a specific engine. (The distributor specifics are commonly listed in the factory workshop manual.) The testing of the distributor to determine its characteristics is most easily done on a distributor test machine, which allows easy observation of what the ignition advance is at any specific engine speed. It is possible, however, to test a distributor on the engine if one has an accurate tachometer and a "dial back" stroboscopic timing light. An assistant or two help to make for easier in situ testing. But, the starting point for everything is: What does my engine require and What does my distributor actually supply?

The other thing to consider is the lack of vacuum advance. While a vacuum advance (an economy measure to help save fuel when crusing) is not needed on a race car set up, it is quite desireable for general driving. Any new distributor should have a vacuum advance unit and that unit needs to be properly tested to ensure that it is to the desired specification as listed in the workshop manual.

As to the question of keeping the original style of coil triggering system (ie-points) or changing over to an electronic coil triggering system, it really makes very little difference. A properly set up distributor having good quality points and a good quality distributor will go a number of years before the points need to be replaced. We used to replace the points about every 12-15,000 miles. How many years does it take most classic car owners to put 12K miles on their car? I have used both points type distributors and distributors with points replacement systems and have found no significant difference over the years.

Finally, there is the fact that Lucas distributors are designed to have recurring maintenance applied at specific intervals, either time or milage. If the distributor is not properly maintained (lubrication mostly), the distributor will not continue to perform adequately and will not operate to the original specification. And, annual maintenance is something that is required, on a regular basis, regardless of the coil triggering system being used with the distributor.

Les
Les Bengtson

Thanks all.

Tempted to have a punt on one of these.
https://www.ebay.co.uk/itm/360500647969?_trkparms=amclksrc%3DITM%26aid%3D1110018%26algo%3DHOMESPLICE.COMPLISTINGS%26ao%3D1%26asc%3D20221025152926%26meid%3Dcd1f5a1049ca48d499e5089d7718a4ac%26pid%3D101584%26rk%3D1%26rkt%3D30%26sd%3D364073301002%26itm%3D360500647969%26pmt%3D1%26noa%3D0%26pg%3D2508447%26algv%3DPromotedCompV3WithHighAdFeeAndToraFilterV1AndItemCfCoviewWithKnnRecallV1%26brand%3DPowerspark&_trksid=p2508447.c101584.m2061&amdata=cksum%3A360500647969cd1f5a1049ca48d499e5089d7718a4ac%7Cenc%3AAQAIAAABMO4A3TcdN9mimaRs6ifRPVr6dZmMdDMbQdtQUZH%252B4LkHdUXRxpZ7xjwVYGDbpJVyFuOWRsZhoDGCZ1%252BJYto8sP2WHcZWNVgvgjXNacO3VOnr%252FZ%252F4duiRbkUZsSpvmjVSEmEob4vJnE3ji6imrAKcOiKDHG%252FePuMEu9ylQQXQ7S4c2a4ufGCdGN4ypsNoJvcu3rBAsBiY6JYxpV4V99hOdvRSR7u8UsJvSLY7cG1dVg11QHUcdVwyNA%252FqAumhmK%252FfoL2bbHN%252FOD0zAdh5j9MYNwe9UhEbfnfXVpGIisQEKs8NLD0tVO5nAQPPMAvUhVU3CcOztZlyui2SFYr0xekUakWm51Z6yzFoA%252FHzC4Ndu1CkTWEsToj0z%252Fbo5Ch9etMfJ%252BpBXYUQdCHupegA%252BvpX9cs%253D%7Campid%3APL_CLK%7Cclp%3A2508447&epid=6021728492

Seems too cheap?
P Peters

P Peters,

You only need the bit up to the ? ie https://www.ebay.co.uk/itm/360500647969

Sounds like one a mate bought recently for about the same price and his came in a damaged box with a poor fitting cap, too tight to remove easily, and a black rotor arm. They did refund him £18 and it seems to run OK so time will tell.
David Billington

Hi

Is your 948cc car a Frogeye or MkII - IIRC slightly OE different spec between the two, noting wise words on here about cars often having swaps in distributors and internal parts.

Have you thought of getting The Distributor Doctor to refurb yours? https://www.distributordoctor.com/

Cheers
Mike
M Wood

Thanks David

Mike, it's a Mk1 Midget GAN1. Engine is a second hand Gold Seal replacement according to a bit of paper in the history.

Don't get me wrong, everything runs fine enough at the moment, so I'm not stranded now. I just want to have a backup plan if a module suddenly fails. So a spare set of points and condensor or spare module if I can figure out what it is. I'll try the cheap one on Ebay I think - get it working correctly and then I have the 23D4 as a spare known working dizzy as backup.

Not looking to improve the engine for now, it's good enough

P Peters

As it doesnt ask what model vehicle it is for, it sounds like it could be a one hat fits all job (see earlier post). Personally I would be nervous about engine-damaging levels of excessive advance unless I could confirm the advance specs.
Paul Walbran

The thing about these new replacement distributors is their quality. I bought one a few years age from a reputable seller here in the States and it was a piece of junk to put it mildly. It lasted about 100 miles with the inside being full of aluminium shavings from the rotor rubbing on the inside of the cap. And the shaft was loose inside the housing with a definite wobble when you moved the rotor sideways. My vote is to get your existing one rebuilt which is what I did by Advanced Distributers in Minnesota. Jeff has a very good reputation here in the US and he has some connection with the Distributor Doctor. They both supply the red rotor.
Martin

I agree with Martin, a rebuilt original Lucas distributor is far superior to cheap modern copies. Another vote for Distributor Doctor or you could try Aldon Automotive.
Bob Beaumont

P. As Paul noted, the advertisment notes all of the vehicles that the Lucas 25D4 distributor was used in, some 35 pages worth. Many, if not all, of these applications used a different specification of the basic distributor, hence the various "specification numbers" stamped into the body of the distribtors. I have seen a very great variation of specifications within the Lucas 25D models--one has a mechanical advance of 10 degrees, the next of 14 degrees, and a third of 20 degrees. (These are "distributor degrees, which is one half of the advance seen at the crankshaft since the distributor rotates at one half engine speed.) There are also some significant variations in the vacuum advance modules.

You know that you currently have a non-standard distributor. What you need to do next is to determine what specification Lucas 25D4 distributor originally came with your engine. Then, you need to look up the mechancial and vacuum advance characteristics of that specification. That is your starting point, and it may be a very good one--the factory thought so.

The absolute best way to set up the ignition system is to have the vehicle checked out on a "rolling road" (chassis dynomometer) and the required mechanical advance curve plotted. Then, recheck how that is working when the vacuum advance is hooked up. (Vacuum advance is used to advance the ignition timing when crusing, allowing better fuel economy.) After you have the plotted curve of the most desireable advance, you either buy a standard distributor, have a standard distributor modifed to the desired curve, or go to some form of programable system set up specifically for your engine.

The easiest thing is to have a distributor gone through and rebuilt to the original specification. Then, you have a good idea that you have a properly set up distributor and it has been rebuilt which, with proper annual maintenance, will give you twenty years of good service before it needs to be rebuilt again.

Les
Les Bengtson

Thank you all. Much food for thought.

P Peters

I bought one from Minispares because it was specifically for use with 285/286/296 cams (I have a 286) AND they specifically say they are NOT mass produced in China. It looks like a 45D but fits where a 25D would fit. It may be the type Alan Anstead mentioned. So far it's been in nearly 12 months and seems OK. It cost more than the one on eBay. Minispares have other choices.
Rob
MG Moneypit

Les
Thanks for the detailed information - I didn't fully appreciate how much advance curves varied. It would be interesting to know how far the advance changes in a used distributor - I presume the springs weaken leading to excess advance.
Bill Bretherton

If a distributor such as 25D4 uses vacuum advance, does it also have the governor weights as per the non vacuum types?

I've used a megajolt and coilpack on a more complex engine in the past and it's really easy to play with the ignition curve. I always thought that simple engines like the A series, when not in a high state of tune, wouldn't be too sensitive to timing within certain parameters.
P Peters

If I recall the weights are the same for the both types of distributor with or without the vacuum advance.
the difference is that the springs which regulate the amount of advance vary between different sizes and manufacture of engine. Lucas supplied the 24/25 type distributor for a whole range of cars from the 848 mini to Mk1 lotus cortina.Hence there are a whole range of springs to suit each application. The amount of advance is also regulated by the advance cam inside the distributor. This is usually stamped on the cam.
Yes a distributor from say a morris minor will fit a midget but the engine may not give its best performance.
Bob Beaumont

Bob, do you mean the weight of the cams? I can see that would make a difference and hadn't thought of it before.
Bill Bretherton

Hi Bill

I think the weights of the cams are all the same (happy to be corrected) it the springs that govern the advance
Bob Beaumont

Bob
Oh I see, I'd read it as though the cams differed. I am wondering now though if I should replace the advance springs although the older ones might still be better than aftermarket. Thing is to measure the advance at higher RPM levels I guess, with a strobe.
Bill Bretherton

To try to address the questions currently under discussion. The addition of a vacuum advance mechanism allows for more ignition advance when the engine is running at a steady state (cruise) which provides better fuel economy. It is not needed on a race engine, but is highly desirable on a road car.

There are two weights which move, as they swing outwards, the mechanical advance mechanism. The weights are pulled back towards the center of the distribtor (reducing the mechanical advance) by two springs. Depending on the desired mechanical advance curve, the springs might be the same size, different sizes, both may be tight (under some tension) at rest, or one might be tight and the second loose. And, yes, the springs will become weaker over the years of use and need to be replaced. Anglo Parts used to sell replacement springs many years ago.

How much mechanical advance the distributor can produce is controlled by an "arm" on the mechanical advance mechanism which, at full advance, is stopped by a post within the distributor. The arm is marked with the total number of distributor degrees of mechanical advance. A 10 would indicate ten degrees of distributor advance which is 20 degrees of advance seen at the crankshaft. (Distributor rotates at one half the speed of the crankshaft. Hence, ten degrees at the distributor becomes twenty degrees when measured at the crankshaft.)

Total mechnical advance is a combination of the "static" advance and the mechanical advance taking place in the distributor at any particular engine speed. Static timing never varies (e.g. 10 deg BTDC) and can be set without the engine running. Mechanical advance varies as the weights move outwards (as the distributor shaft sping), eventually hitting some form of maximum mechanical advance when the cam stop arm hits its stop post and the weights are no longer allowed to move outwards.

Weakened springs can allow the mechanical advance to increase too rapidly. Gummed up internals can cause the parts to move sluggishly (sometimes not at all). Lack of lubrication, worn posts, and worn holes in the advance weights can all have an effect on how well the distributor works. That is why I prefer to start with a new distributor or one rebuilt by a competent craftsman.With either, it is wise to obtain the use of a dial back stroboscopic timing light and actually check the advance curve against the factory specification. Be aware that the early curves given in the workshop manual tended to be absolutes (e.g. 15 deg advance at 1200 rpm) which later specifications were presented as a range (e.g. 15 deg advance at 1200-1500 rpm). You probably need to be close to the factory specification, but a degree or two might not matter much on a basic engine but may become more important in a race/rally engine.

As to the distributor that looks like a Lucas 25D4 but uses the 45D4 parts, I purchased some several years ago. They were sold as the Lucas 45D4 "Euro-Spec" distributor. The Lucas 25 and 45 distributors will physically interchange, with the 45 having a better system for the points and condensor, as well as a larger distributor cap which makes it less likely to cross fire due to carbon tracking inside the dizzy cap. But, the advantage with a four cylinder engine is not as great as when the distributor is used with a six or eight cylinder engine. In the latter cases, the 45D is a very significant improvement over the earlier models.

Les
Les Bengtson

Yes, the springs are it, the only differences in the weights and cams are the length of the stop on the cam which governs total advance.

A series engines are just as sensitive to advance as any other engine. In general on anything, the higher the effective compression ratio (combination of CR and inlet valve closing point) the touchier an engine will be about advance, especially if the fuel octane is lower than optimum.

We do a lot of distributor work, and have found that it isnt so much the springs weakening as that they wear in the hooks where they attach to the posts, and also the posts themselves wear. Or someone has swapped in another distributor. And yes, to diagnose it is essential to check timing with a strobe right through the speed range until advance stops.

The wear occurs most in the secondary spring. For those not familiar with how they work, only one spring is under tension at lower speeds, the other having a pre-determined bit of slack which takes up as engine speed rises. Once taken up, the advance rate is slowed until it finally meets the stop. When plotted against RPM this results in a "curve" of three straight lines, the last of which is horizontal. (Earlier DM2 etc distributor have a different set-up which relies more on geometry changing, and do give more of a "curve".)
The secondary spring wears more because it will chatter a bit at the take-up point. In the earlier MGB engines this occurs at idle, which seems to worsen the wear.

The 23D and 25D are identical in operation other than the vac advance. 45D has the same principle but the design of weights and stops are different (and also better balanced as the stop is not so assymetrical). But there are large variances within each type of the rpm and amount of advance when the second spring takes up, and again where it hits the stop. I have a Lucas book which sets out hundreds of widely different combinations of this as used for a large range of cars. To pretend that just one version can do all those is misleading and delusional. Fitting the wrong version will at best only affect fuel economy, at worst will damage the engine.

The vac advance varies similarly. If a wrong vac advance is fitted it too can cause loss of economy or even damage in the long term if advancing too soon or too far. This latter situation is characterised by pinking which occurs on light throttle openings and disappears at full throttle when the vacuum drops out.
Paul Walbran

Les, Paul
Thanks for the detailed comments - very interesting.
Bill Bretherton

This thread was discussed between 03/05/2023 and 09/05/2023

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.