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MG Midget and Sprite Technical - Lucas distributor

I am in the final stages of my Bugeye rebuild and have a question on the distributor,The original DM2 is missing and seem to be near impossilble to find. However I have a good 25D (40767 f) This Looks to be very simalar,will it do the job and what are the differences?
Noel Copping

Here is a DM2P4 distributor on ebay

http://cgi.ebay.com/ebaymotors/NOS-LUCAS-DM2P4-SUNBEAM-Rapier-Alpine-1959-61-40683_W0QQitemZ360108906140QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item53d829d69c&_trksid=p4506.m20.l1116

The bentley manual lists this as original specification for all spridgets with the 948cc engine. The 25D does look similar but looks dont matter nearly as much as the advance curve. Just looking at the bentley manual I noticed that the stock static timing set up for the 948 was a mere 4 degrees btdc. I don't know what the curve would look like on the original set up, or even wht the curve is in your 25D. The first thing I would do is make sure the 25 D actually will fit in the engine you have and that the drive dog will engage the non original distributor. Next I would make the decision wether i was super worried about originality, or if I didnt care what distributor went in so long as it worked.

Next, I would contact the boys at Aldon automotive and ask for advice based upon your current set up. Then I would send EITHER the original distributor or the one you have, in for recurving to suit your needs.

Of course, if you're not real fussy, you could just stick it in and see what happens
S.A. Jones

LOL...S.A.

>>>>>>>>>Of course, if you're not real fussy, you could just stick it in and see what happens <<<<<<<<<<

That was funny, I enjoyed the country side ride you took us on for finally getting to the the best advice for your response....classic LOL


Hey noel,

Im Not fimaler with the "DM2P4 distributor" and I dont remember alot of discussion on this paticular dissy. But if you type in that same phrase into google there is alot of information there...If anyone can offer more info then S.A. my money is on "Les" out of ariziona...try posting a thread looking for his help, he is usually on this board around once aday to as long as 2-3 times a week

good luck with that
prop
Prop

Actually MkI Sprites (Frogeyes) had the Lucas DM2PH4 distributor.

It was the MkII Sprites and MkI midgets (948cc) that had the Lucas DM2P4.

1098cc cars had the 25D4.

1275cc cars usually had a 25D4, although some early cars had the 23D4 (no vacuum advance unit).

Unfortunately it isn't as easy as buying that EBay distributor from a Sunbeam Alpine/Rapier since the advance curve will, in all likelihood, be radically different - in the same way that you can't swap a Midget 25D4 distributor for a Hillman Imp 25D4.

What will happen is that of the advance curve is wrong for the motor you will likely get pinking unless you back the static advance right off - and then you'll loose out on power at the top end.

Even if you rolling road the car and set the ignition for MBT it will still be out.

As has been suggested, I drop the boys at Aldon Automotive a line and see what they have to say:

http://www.aldonauto.co.uk/shop/contact.asp?

AFAIK all their A Series distributors are variations of the Lucas 45D4 (we have one on the race car) which, unfortunately, doesn't help from the point of view of originality, but if you are willing to accept that then they could sort you out.

Alternatively, if you are worried about originality then contact Martin Jay at 'Distributor Doctor':

http://www.distributordoctor.com/contact.php

He rebuilds all types of Lucas distributors from the 1940's to the 1970's. I would think he could sort your problem although I am guessing he will levy a surcharge because you don't have a unit to exchange.
Deborah Evans

BTW, if you do go with the ebay item, just keep in mind that that particular distributor was set up for a sunbeam "rapier alpine". As such, it has a worm ger on the end of it which would mate directly with the cam on that engine rather than with the keyed end of a drive dog on an aseries. Theorectically you should be abe to remove this worm gear and install the proper key, but that may be best left to professionals. Very likley the curve wont be exactly right for a 948cc sprite but aldon could fix you up if you sent it in.

Again, if you arent real fussy and you get this distributor, press the key off your 25D (usually a pin somewhere in there) using a homemade "drift" a benchvice and a beer. Then stare at the worm gear on the DM2P4 and scratch your head a few times while you try to figure out how those engineers in england managed to get that thing on there and maybe even what pub they went to and what they were drinking when they designed it. Next, once you know how it went together, take it apart using whatever you have at your disposal that might distroy the worm gear but leave everything else untouched.

Finally, gaze upon the key from the 25D and the now wormgearless DM2P4 and scratch your head a bit while you figure out how to get the two together. Afterall Lucas made distributors for a variety of applications and each type should have interchangible drive systems right? Only makes economic sense. The key SHOULD fit. Just have to get it on.

Failing that, send it to aldon and let them deal with it.
S.A. Jones

Thanks for your response to my question,The 25D has the correct drive dog, and I suspect it came of a latter model Sprite,I realise that the advance curve may very ,but how different would it be for a latter sprite given that the static timing for all models are much the same,and the dynamic figure only a couple of deg different.
Noel Copping

My guess it would be influenced by smog regulations...aka the smog equippment....I cant tell you thats the case or send you to any articals, but thats my opinion....Btw my opinion is worth $19.95..if you can drop that in pay pal that would be cool...LOL.


Prop
Prop

Prop
From what data I can find my 25d (40767) distributor is of either a 1098 spriget or a 850 mini,therefore Idont think there would be any great difference in the advance curve.As we dont have the smog equipment bullsh*t requirement on our cars down here,very little would change between models. I gess I,ll just fit and see what happens.

Dose anyone know where I can find a ref.or listing as to the Lucas distributor model numbers. ie 40767.
Noel Copping

Noel. The following website has a Lucas distributor data base: http://www.starchak.ca/tech/pdfs/lucas.pdf which may be of use to you.

I checked out the Lucas Data base and it noted that this distributor was used in the "Mini Commercial Models, Moke, and Special Order". The advance curve listed was:

3,000 rpm 13 deg of advance
1,950 rpm 8.5 deg of advance
850 rpm 1.5 deg of advance

All of these numbers have been taken from the Lucas specifications which means that they are the specifications which one would read from a distributor test machine. The distributor rotates at one half the engine speed. So, to convert these figures, as observed on the dizzy test machine, into figures observed at the engine's crankshaft pulley, multiply by two.

Not sure that these figures, designed for use in a Moke, which is kind of like a Mini pick up truck, used for hauling big loads and biased towards low end torque and lower engine rpms, are what would be of interest to us as drivers of sporting cars. (I must apologize here. I am, in my advanced years, and MGB driver. Thus, not a sports car driver in the sense that a Midget owner or a Sprite owner would recognize. But, when young, I owned a Mark II Sprite, two MGB 1600s and an Austin-Healey 100. So, I know what I am missing should I ever be limber enough to get into a Sprite again.)

Deborah has provided a quite useful link to the "distributor doctor". Thank you, ma'am. I will pass this information on to others. The MGB owners have never mentioned this source. Again, thank you.

Another source of distributor rebuilds/information is Jeff Schulmer (sp?) who posts here from time to time. His proper name, and his company, were posted on a recent thread on the MGA board. I have corresponded with him and believe that he knows what he is doing and takes this subject seriously. People who have used his service report that they are quite impressed with the quality of his rebuilds.

I would recommend you contact several informational sources. Tell them what you want out of your engine, what the current state of your engine is (has it been rebuilt, what cam, any cylinder head work, any changes from the basic factory specification?) and what the available fuel is (here in the US, we get these "blended fuels" which are not particularly fuel efficient and the octane rating keeps dropping).

They will give you their ideas of what might work best for you. Something which will provide adequate performance with little chance of engine damage. After that, you can set up a "rolling road/chassis dynamometer" session to ascertain the best possible ignition advance curve for your particular engine. Should you decide that such an advanced degree of tuning is necessary. Most of us do not require such unless we are racing or participating in advanced rallies.

Les
Les Bengtson

Noel,

There is nothing I can provide beyound what les and deb have provided, both are considard to be the cream of the crop. togather they rep. about 80 years of experiance between them

Prop
Prop

Les
Thanks for that valuable Lucas data, this of great help ,I have Taken the data for the original DM2 sprite distributor and drawn it out on a graph along with data the 25D that I have,and data for several other A series model and they all give a very simalar curve,and if anything a little retarded from the original spec,which with todays fuels should not be a problem,and if need be I can advance the static setting to compensate. Is this sound logic or am I missing something?
Noel Copping

Les, thanks for posting that Lucas link, VERY much appreciated. Now stored for future reference. :)

Noel if the advance curve is slightly retarded then advancing the static advance will not change the curve but will change the total advance at different points on the curve as well as the total advance seen.

Taking Les' figures (which are also for early 850 Mini):

3,000 rpm 13 deg of advance
1,950 rpm 8.5 deg of advance
850 rpm 1.5 deg of advance

At the engine this equates to:

1.5 degrees @ 1700 rpm
8.5 degrees @ 3900 rpm
13 degrees @ 6000 rpm
Advance starts from 850 rpm

The curve for a Frogeye with the DM2PH4 distributor would be:

1 degree @ 1200 rpm
6 degrees @ 2000 rpm
13 degrees @ 5600 rpm
Advance starts from 900 rpm

As you say, the 25D4 curve you have is similar but a little retarded.

Dial in extra static advance and you will (effectively) improve the middle part of the curve but you may well then run too much total advance at max rpm. Not so much of a problem at max rpm since you will be at WOT but you may see a drop off in power.

For eg, a number of fellow racers with 1380's run around 30 - 32 degrees max advance, whereas, for max power we run 29 degrees (from the rolling road data). Above 29 degrees on our engine and power starts to fall off markedly.

I would be more worried about the middle range partial throttle settings (especially on crappy modern fuel) since we don't know what the vacuum advance settings are.
Deborah Evans

"I would be more worried about the middle range partial throttle settings (especially on crappy modern fuel) since we don't know what the vacuum advance settings are."

For that, all ya need is a mityvac to test and see just how far the vac advance moves things.
S.A. Jones

Deborah

Just looking at your figure for 6K rpm: 13 degrees at the crank doesn't seem to be a fat lot of advance - adding the 7? degrees static gives only 20 degrees.

Shouldn't this be nearer 30 deg, or are the early engines so different from late?

A
Anthony Cutler

A. Minor mistake in Deborah's math here. She properly doubled the figures to convert distributor rpms to engine rpms, but did not double the degrees of distributor advance to equal what you would see at the crankshaft pulley.

At the engine this equates to:

1.5 degrees @ 1700 rpm
8.5 degrees @ 3900 rpm
13 degrees @ 6000 rpm

These figures should be:

3 deg at 1,700 rpm
17 deg at 3,900 rpm
26 deg at 6,000 rpm

For comparison, double the mechanical advance figures of the Frogeye dizzy in the same manner. Also remember that the early specifications were listed in this manner--a certain number of degrees at a single specific speed, while later specifications would specify a certain number of degrees of advance over about a 500 rpm range--e.g. 17 degrees at 1750 to 2250 rpms.

Were it my car, I would try out the current Lucas 25D4 distributor and see how it worked. I do not believe it would damage the engine to operate it with that distributor. I would book myself a rolling road, were such things commonly available where I live, and let them determine what might be the actual advance curve of my distributor and to determine what might be the best advance curve for my engine. These things vary and old distributors might not show the factory advance curve any longer.

As to the vacuum advance, that is seldom a significant problem. It comes into play when one is driving along at a steady rate and the engine is not under load. It provides additional ignition advance which allows the engine to operate, under certain conditions, slightly more efficiently and using less fuel to do so. A functional vacuum advance unit is desirable, but its exact specifications are not as important as the basic mechanical advance curve.

Les
Les Bengtson

Sorry about that. Les is quite correct - I got distracted and forgot to double the advance figures.

Mea Culpa.
Deborah Evans

This thread was discussed between 30/05/2009 and 31/05/2009

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