25D distributor types

Post by **ndevans** » Sat Nov 28, 2009 12:37 am

OK folks another brain teaser for you.

The distributor on my engine (a 1098, prefix 10MA-U-H, from 1969) is stamped 41257 B 2872. The weights inside are stamped 10, I think this refers to 10 deg mech advance.

I know the 2872 means week 28 of 1972, indicating that it's not the original distributor. Looking in one of my Minor manuals, there are type no.s for the distributors fitted to Minors, SV engine, OHV low comp & high comp. 41257 does not appear in the list.

I also have a spare distributor that I have been given. This one is stamped 41124B 1065, so from Oct 65. This one has weights stamped 13 deg. Looking in my manual, 41124 IS one of those listed, for an HC Minor.

So what engine is a 41257B Lucas 25D for? And what effect, if any, does it have on my engine? Am I better off with the 41124B unit?

cheers,
N

Post by **mike.perry** » Sat Nov 28, 2009 1:19 am

Search for Lucas Distributor info under Electrics and you will find a site with all the dizzy codes and applications. The side valve engine uses a DKH4A distributor which does not have a vacuum advance unit

Post by **ndevans** » Sat Nov 28, 2009 8:36 am

Hmmmmm. Thanks Mike.

Neither 41124B or 41257B are listed, however 41124A & 41257A are. 41124A is for Austin, Morris, Minor & Traveller, 41257A is for 1275 Mini etc. So at some point someone has replaced the standard unit in my Minor for one off a 1275 Mini.

Think I might try the 41124 to see the diffference.

Cheers N

Post by **mike.perry** » Sat Nov 28, 2009 8:40 am

I doubt if you will notice, a lot of the variations are to do with emmisions and you would probably have to use a rolling road to compare.

Post by **ndevans** » Sat Nov 28, 2009 8:43 am

Useful to have a working spare though. One difference between the two is the cam lobes on the 41124 are much sharper than on the one off the car. I presume this will have an effect on the dwell angle?

Post by **ssnjimb** » Sat Nov 28, 2009 9:30 am

just a shot in the dark (B-B Series)

It could have been replaced when the car was in for repair or service at a morris dealer

Jamie

Post by **ndevans** » Sat Nov 28, 2009 10:31 am

Quite possibly. It certainly hasn't been done while the car has been in the family, since 1986.

The car runs sweetly enough, you can't complain about a 40 yr engine that can still deliver 45mpg on a run!

I'll give the other one a go, if it's worse then it's easy enough to swap back.

Looking at the data in the pdf on the website that Mike gave earlier, the difference between the two is:-

Serial RPM1 Advance1 RPM2 Advance2 RPM3 Advance3 No adv below
41124 3400 12 1500 4.5 800 1.5 300

41257 2500 10 1400 6.5 400 0.75 150

Vacuum advance

41124 5/8/3 (z)

41257 3/10/10 (y)

Dunno what the vac advance figures mean or what the z & y means, if anyone can explain I'd be grateful.

linearaudio · Post by **linearaudio** » Sat Nov 28, 2009 5:29 pm

The dizzy with the softer lobes will allow you to rev higher-the opening ramp is same, but the closing ramp is longer to allow the contact heel to follow it rather than bounce at higher revs. You won't notice any appreciable difference between the two.

Post by **bmcecosse** » Wed Dec 02, 2009 6:12 pm

Yes ^^^^ you want the 'softer' assymetric lobes - but also you want the 13 degree advance (which is really 26 engine degrees) - and a vacuum advance unit that WORKS. The numbers on the vac do mean (in some way) the amount of advance - and the vac figure when it comes on - but i don't know the codes. Just be happy to have one that works!

Post by **Alec** » Wed Dec 02, 2009 6:59 pm

Hello Neil,

there are dozens and dozens of different 25D4 distributors but in the past they would just be swapped with one that looks the same with no regard to it's settings or whether it suits you engine. There are the same variations, and they are quite significant, between vacuum modules. The figures, are vacuum at which it starts to operate, vacuum which gives maximum advance and lastly the engine advance in degrees (not distributor advance, which as BMCE points out is half engine advance) So you see that the two you quote, there is a seven degree advance difference between the two, I would call that significant.

Hope that helps.

Alec

Post by **bmcecosse** » Wed Dec 02, 2009 11:01 pm

I suspect that any 'new' vacuum units you buy these days will just be a compromise. In any case - we now have 'unleaded' fuel - and all these older units were 'designed' in the days of a very different 'leaded' fuel.

Post by **Alec** » Thu Dec 03, 2009 7:41 pm

Hello BMCE,

I don't know what replacements are available but it is difficult to compromise when units are originally of such varying specifications. Certainly the fuel is different, unless you chose to buy leaded at the few outlets that still sell it, but I suggest that the engine's ideal curve will be the same but shifted by X degrees from the leaded figures?

Alec

Post by **bmcecosse** » Thu Dec 03, 2009 7:57 pm

No idea Alec - but I doubt there is a range of new vacuum units being made now - if any! Maybe all new-old-stock - and we just have to take whatever we can get our hands on!