Vacuum Advance Unit on an early 1974 MGB - adapt to Ported or 3rd party solution?

[Bourne]

A few years ago my distributor gave up the ghost and I switched to an Accuspark electronic ignition. My old distributor was a 45D4 but as the books all indicated the car (an early 1974 MGB) would originally have been fitted with a 25D4 I ordered a replacement in that shell.

Everyone can probably guess where I’m going with this. All I can say is that I got an MGB to learn!

My car was manufactured in the brief window when cars were still being fitted with 25D4’s, but the connection for other end of the vacuum hose had switched from the Carbs to the manifold. Earlier this summer my car started idling at lower revs than optimal and stalling occasionally at junctions. Once I’d replaced fuel lines, cleaned the electrical connectors to the starter solenoid, and topped up the dash pots (well I did get the MGB to learn!) I eventually checked the vacuum advance and confirmed it is ruptured.

The internet indicates I need a vacuum advance unit with Lucas product code 54425359 and a vacuum code of 10-15-5. This seems to be a rare part to the extent that I can’t see one available in the UK. I have emailed Distributor Doctor as I suspect he is my best chance for sourcing the part. But there are no guarantees.

That leaves two other options for which I wanted to seek advice on this forum:

1) Are there any other third party 10-15-5 vacuum units which would fit a 25D4? (I assume not - at least those anyone would recommend)

2) Could I buy an easy to source vacuum unit for a ported vacuum setup and connect the hose to my carbs. Is this as simple as changing where the hose is connected? Are there disadvantages to using a ported vacuum setup?

As always - Thanks

[Ian F]

Just a very brief reply - I am sure that the DD will be able to provide a complete unit to meet your requirements. The 25D4 is a very common piece of kit so there must be lots of spares suitable for rebuilding.

Ian F

[Bourne]

Thanks Ian,

DD has come back to me. He believes the car would have been fitted originally with a 45D4. This indeed was the distributor fitted to my car when I bought it, but I had assumed at some point the distributor had been replaced.

MGB stuff https://www.mgb-stuff.org.uk/distributors.htm indicates however that a MGB manufactured in July 1974 would fall in the brief window when cars had switched to manifold vacuums but still used 25D4 distributors.

I have also consulted Clausager. It states 45D4 distributors were introduced with the Rubber Bumpers in September 1974. This tallies with MGB Stuff. I can not however find any reference to the switch over of distributors in the production changes chronology.

So the authoritative sources I trust don’t tally with each other and I will need to keep digging. In the short term however, MGB Stuff suggests that the precise vacuum advance unit isn’t that important https://www.mgb-stuff.org.uk/distributors.htm#vac. This surprises me as it seems to go against the collective wisdom of the Internet. But it’s given me enough confidence to order a vacuum unit for my current 25D4. We’ll see how that goes when it arrives.

Regarding the switch over from Manifold Vacuum to Ported Vacuum. It appears this is technically possible but would involve retuning both the ignition timing and carb balance. It’s still an option but I hope MGB Stuffs steer on the Vacuum Advance Units means it won’t be necessary.

[Ian F]

Thanks David, that is a lot of research! I would guess that you now know more about this change point in MGB specification than most folk on this forum!
I think I might have taken the easy route and just swapped for a 25D4 and associated manifolds etc. I guess that might have been a costly option.

Best regards

Ian F

[Michael Barclay]

Hello David
If your car was built in 1974 it would have been fitted with HIF carbs originally. These were introduced on UK and European cars in Nov 1973. With HIF carbs the Vacuum advance is taken from the inlet manifold rather than the rear carb as on HS4 Carbs. The distributor fitted will depend on the engine fitted to your car as this was the period when they changed from 25D TO 45D Distributors.
18V779 and 780 Engine would be fitted with 25D Distributors .18V846 and 847 Engines would have a 45D distributor.
It was all about trying to get the B engine through the ever-tightening Emissions regulations.
Mike

[Bourne]

Thanks both

Yeah. Fully conscious that this is rather a niche question. On the other hand, I trust the collective wisdom of this Register’;s forum over the dubious algorithm chasing videos that are on certain social media platforms.

Thanks David, that is a lot of research! I would guess that you now know more about this change point in MGB specification than most folk on this forum!
I think I might have taken the easy route and just swapped for a 25D4 and associated manifolds etc. I guess that might have been a costly option.

Best regards

Ian F

Thanks. Given that the distributor itself is relatively new and works well; I’d be keen not to repair the broken component rather than replace the unit. Although judging from Michael’s point below about the Carbs being designed for Manifold, replacing the unit, if ultimately necessary, would seem sainer than trying to switch to a Ported vacuum.

I ended up buying a cheap generic Vacuum Unit from one of the reputable stores. It doesn’t have a vacuum code stamped on it (then again neither does the one it’s replacing) but appears to be functioning properly now it’s installed (I’d forgotten how well my car starts). Hopefully it will last. If not, I’ll acquire a refurbished original from the Distributor Doctor and update the thread.

Hello David
If your car was built in 1974 it would have been fitted with HIF carbs originally. These were introduced on UK and European cars in Nov 1973. With HIF carbs the Vacuum advance is taken from the inlet manifold rather than the rear carb as on HS4 Carbs. The distributor fitted will depend on the engine fitted to your car as this was the period when they changed from 25D TO 45D Distributors.
18V779 and 780 Engine would be fitted with 25D Distributors .18V846 and 847 Engines would have a 45D distributor.
It was all about trying to get the B engine through the ever-tightening Emissions regulations.
Mike

Thank you. My car was indeed originally fitted with a 780 engine (the current engine is a Gold Seal) so that tallies with my research that the distributor would have been a 25D. I suppose it is possible the 45D was used when the Gold Seal was installed in the car (late 1970’s from my memory of going through the car’s history) but I think its more likely its a later replacement and the owner at the time went for the thing that was closest to hand.

The good news though is the car is running well again.

[Not_Anumber]

Its great that has got the engine running smoothly again. Have you got any more work on the car planned next ?

[Bourne]

Its great that has got the engine running smoothly again. Have you got any more work on the car planned next ?

Oh Goodness. Now the car is actually running. I think cosmetic projects for the time being! Although…

…see my other thread