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MG MGB Technical - Help identify this 25D distributor

I needed to replace my worn 45D4 distributor (shaft wear causing misses), so I purchased a rebuilt one from a local autoparts store - specializing in european parts.

My car:
1977 B
desmog
stock cam
Weber downdraft
163-165 psi per cylinder
(let me know if you need anything else)

Anyway, the store provided me with a 25D.
The numbers on the unit:
41199 D
17 -71
On the vac:
4-7.6.54411983
On the advance weights:
18 deg

I searched the archives, (and Paul Hunt's page), and can not find this unit (41199). Is this a suitable replacement- or even made for my car?

I appreciate any help.
Tony
Tony Calleja

Tony. The only list I can find that shows that distributor does not list it's application. It shows a 41199A and a 41199B, both of which show a 15 degree (30 at the crankshaft) advance at 2,500 rpm. Your advance weights are not, normally marked, but the cam arm is. In any event, 18 deg dizzy advance is 36 deg crankshaft advance. This system might work if you take the vacuum for the advance off the carb. If, however, you take it off the intake manifold, the timing will probably be somewhat retarded. I have never seen anyone have a dynomometer session with a 76-on car to plot the ideal centrifugal and vacuum advance curves. The factory system was designed for emissions control, not performance. Considering the fact that Peter Burgess recommends 32 degrees total advance, and we know that lower compression engines (such as the 18V engine) require somewhat more advance, I would give the new dizzy a try and see what happens. If you get pinging, retard a few degrees and see what happens. Les
Les Bengtson

On a related thread, does anyone know if the vacuum source is different for a '72 or '73. I replaced my 25D on my '73 with a '72 and am having a huge problem getting the thing timed.

On another note, I found a complete list of lucas distributors online. The list is at home and I will post it as soon as I get home.

thanks
Zach

This is an adobe file and when I tried opening it again under my computer it made me download it, but on the old computer I could just open it as a webpage. Also it may take a little while for the page to load, it is a large file.

http://www.telusplanet.net/%7Echichm/tech/lucas.pdf

Zach
Zach

That info is far too valuable to leave on the web, it could vanish anytime. I downloaded it anyway.

Looks like North America changed to manifold in 71, so both yours should be the same. What problems are you having? Fluctuating readings? Pinking? Poor performance?
Paul Hunt

My original distributor was missing part of the collar that was used to hold the distributor in place. I purchased a used one from someone to replace it with.
I made sure that it was for an 18V engine and then swapped my pertronix, rotor button and cap from my distributor. I then set the static timing to 10 degrees as I had done with the previous distributor.
The idle jumped to 1200 instead of the old ~850 that it was set before. So I tried turning the distributor body (with the vacuum advance unplugged and the intake port plugged) to get the rpms more reasonable. Then it starts shaking as if there was a vacuum leak. I turned it back so that it idles at 1200 and took it out for a test drive (a little anxious to drive it again). On deceleration in gear, the car backfires horribly (no backfire when not in gear). I think that this means that there is too much timing. It also was coming very close to overheating. I pulled the plugs and the electrodes were slightly tan indicating good mixture.

In general it just runs too rough (car shakes when idling). I am not sure what I could have done that would have changed anything? I have no had a chance to check for vacuum leaks, but that will be on my list to check this weekend, could there be a vacuum leak at the distributor (ie would it matter if there was)?

A little confused and anxious to drive. Thanks again.
Zach

Thanks to everyone.
(Zach - I have a new reference manual)
(Les, when the engine was rebuilt three years ago, HC pistons went in. Also, I'm taking the vacuum off the carb)

I installed the distributor and drove around today. The car ran well, no noticeable ping until the engine warmed up beyond the 1/2 mark. Then it pinged slightly on acceleration. I retarded the timing a bit more (to 10 degrees) no noticeable ping so far, but the idle does seem a bit "lumpy". Guess I have more adjustments...

Is the advance curve the main difference between distributors (appears to be)? I mean, if you take off the vacuum and advance weights, are all 25D units the same? If so, how come it's so hard to find various weights to fine tune?

Tony
Tony Calleja

Tony - as well as the vacuum capsule the main difference between the different serial numbers is the springs, and these determine the advance curve. On many types the weights are common and the max advance is set by specific machining of the cam plate and in these cases the max (distributor) advance is stamped on the cam plate, on others the cam plate is common and the weights differ and these have the max advance stamped on the weights.

So many years after going out of production it is not surprising that these parts are hard to come by. There are 'rebuilders' who will put any old parts inside and pass them off as a specific serial number, there are others who will tweak what springs they have on a timing machine until they get the required curve. Caveat Emptor.
Paul Hunt

Just as an aside, Marcel Chichak's website (referred to 5 posts above) has some other interesting information related to LBCs.
Marvin Deupree

This thread was discussed between 04/09/2002 and 07/09/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.