Morris Minor Owners Club

I wasn't sure where best to post this so have put it under general discussion?

I was checking out the ancillaries for my 803cc engine rebuild and found that the Lucas distributor vacuum advance unit was no longer holding as its rubber diaphragm is now 75 plus years old. I think its a type 25D distributor?



I looked at replacement vacuum units but they looked very expensive and also would very likely have very old rubber in them.
I decided to try and rebuild my original unit so opened it up for inspection!


First the unit was removed from the distributor and then the intake brass union was unscrewed and the spring and plunger were then freed.


A small sharp screwdriver was used to start opening up the peened over flange on the casing then a worn larger screwdriver was used to gently release it all the way around a little at a time until the alloy part could be withdrawn. Taking care not to overstretch or split the steel.

Once opened it was obvious why it was leaking.....also the rubber diaphragm was getting a little crispy!

I had a measure and then looked for replacement material..I found 0.5mm Nylon reinforced Nitrile Diaphragm fuel pump sheet for around £10 on ebay ...enough to do a few replacements.

The rivet securing the diaphragm was drilled off and the old diaphragm removed. The riveted part will be replaced with a tapped 2BA thread and screw loctited into the shaft

I will post an update once the new diaphragm arrives.

Taupe

I don’t think that’s a Lucas 25D4 but I can’t tell you which model it actually is. Try typing the Lucas number stamped on the side of it into Google to identify it.

There are books with tables in them (which I don’t have) which have the specifications for the specific distributor variants for different engines/cars. They specify the advance curve (the degrees of advance a different rpm) and the vacuum advance at specific engine vacuum in mmHg (millimetres of mercury). Distributors may look fairly simple but for an engine to run as intended they require a high degree of technical knowledge (the reference tables) and testing equipment which the hone mechanic is not likely to have. That’s why getting a distributor rebuilt by a professional is expensive.

Stephen

Thanks Svenedin

Its a DM2 I think not a 25D

Yes "getting a distributor rebuilt by a professional is expensive" and not worth doing when you can buy a new replacement electronic one complete for around £40-50

eg 2
Lucas Distributor

exchange. DM2 type, top entry,

100M
Product number: 294153
Required: 1
Regular price: £512.98

Thats why refurbishing the original unit is so attractive ....cheaper and it will be correct technically ..matching the original specification exactly

Taupe

Taupe.

But do you have a Blue Peter badge ?

John. ;-)

taupe wrote: ↑Mon Oct 05, 2026 9:26 pm Thanks Svenedin

Its a DM2 I think not a 25D

Yes "getting a distributor rebuilt by a professional is expensive" and not worth doing when you can buy a new replacement electronic one complete for around £40-50

eg 2
Lucas Distributor

exchange. DM2 type, top entry,

100M
Product number: 294153
Required: 1
Regular price: £512.98

Thats why refurbishing the original unit is so attractive ....cheaper and it will be correct technically ..matching the original specification exactly

Taupe

The Chinese electronic ones are utter garbage. Generic advance curves not matched to the engine, badly made, no functioning micrometer adjustment, timing wandering all over the place. I tested one and promptly took it off the car and had my original distributor rebuilt by Distributor Doctor. If you want it you can have it! It’s positive earth though

Stephen

I looked up the codes for the early series 2 DM2 dizzy and its 3/13/12 on the vacuum unit....mine is stamped 3/13/1 but I think maybe the 2 on mine was probably poorly stamped or omitted

This code means:

Understanding the 3-13-12 Code

• First Number (3): The vacuum level in inches of mercury ("Hg) at which the unit starts to function/advance.
• Second Number (13): The vacuum level in inches of mercury ("Hg) at which maximum advance is reached.
• Third Number (12): The maximum advance provided in distributor degrees (which translates to double that—24 degrees—at the crankshaft).

I have a mercury calibrated vacuum gauge so will be able to check calibration when completed but I cant see why its settings would change by just replacing a diaphragm!.

(Rob Thomas posted some details on this same dizzy in May 2017)

Taupe

That's great that you have been able to find the figures. I am not sure exactly how the diaphragm itself influences how the mechanism responds. I suppose if the diaphragm material is too stiff it would have an effect. Looking on the internet there seems to be misunderstanding of the vacuum advance/retard, getting it the wrong way around. As far as I understand it, when the vacuum is high (such as when idling) the mechanism advances the ignition. When the vacuum is low the mechanism no longer advances i.e retards it. This is like having an automatic system to do the job of the advance/retard lever on the steering wheel on pre-war cars. Advance at idle or when cruising at light load, retard when engine vacuum is low such as climbing a hill (this is also when an engine might pre-detonate/pink from over-advance).

Thanks for posting the insides of the vacuum unit, I've never seen that. Somewhat reminiscent of the insides of my Smiths PCV valve.

Stephen

The diaphragm material arrived today...its pretty much identical to the original material.

So I marked out first

Then cut out a disc with sharp scissors (just slightly larger than the alloy diameter)


Then drilled and tapped for a 4 BA screw (the head using a 2BA is too large to clear the push rod)



The pushrod and spring now fit over the 4BA screw head

I think that the pushrod length controls the maximum amount of advance in conjunction with the number of alloy washers fitted under the brass union nut.


Then the screw was thread locked after the diaphragm and small washer were fitted (a hole was punched in the centre of the diaphragm first for the screw)

The arm is realigned with the dot on the casing so that when peened closed it is correctly aligned (this is important!)

Just the final reassembly, peen over and testing to do now!

Taupe

Excellent work and will be very satisfying when fully working

Thanks Bill

All back together now and it holds a vacuum without leaks.

I freed up the spindle on the advance mechanism this afternoon within the dizzy as well and it all looks good with minimal wear in there so it will soon be ready to refit once I complete the BRITMO 803cc engine rebuild.



I will give the steel cover a lick of silver paint after doing some performance tests on it.

Taupe