XJ6 distributor with vacuum retard or vacuum advance?

Hello all,
I have reached another cross roads.
I have 2 distributors on my table and I need to choose one to finish the engine assembly and start it next weekend.
The original one is the vacuum retard type, the spare one is the vacuum advance type.

What is a man to do?? :slight_smile:

The car is a 1975 XJ6C, new everything inside, now non-original, double HIF7 carbs, smog-test exempt, all smog equipment removed.

Thank you.

Interesting article: http://www.georgiajag.com/Documents/Vacuum%20Advance%20versus%20Vacuum%20Retard.htm

Never understood the logic behind vacuum retard, Eric…:slight_smile:

Whatever; the xk in the European set-up, with vacuum advance, is the original concept and the more powerful - since no emission control requirements were in force.

You just have to ensure that you set up the engine to follow one or the other spec…

xj6 85 Sov Europe (UK/NZ)

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Bywater once wrote an interesting treatise on it. IIRC, it could be summed up as follows: With a vacuum advance system, you get part of the advance you need from the centrifugal and part from the vacuum. With a vacuum retard system, you get all the advance you need from the centrifugal and the vacuum takes some away as required. Set up properly, there’s less difference than you might think – which is to be expected, as optimum advance is optimum advance regardless of the mechanisms used to achieve it.

That said, there is a lot of confusion and misinformation about vacuum retard systems. I think you’d have much better chances of getting a vacuum advance scheme set up properly than a vacuum retard scheme.


Adding, Kirbert, that ‘retard’ is associated with ported vacuum - which in itself has a somewhat convoluted vacuum curve. Generally set up with less advance to ensure against detonation - which reduces power output.

Way back in Europe; tax were paid on nominal horsepower, based on engine volume. Henceforth one tried to squeeze maximum power out of minimum engine volume - so vacuum advance was the story. US had the advantage of just putting in a bigger engine to gain more power - and then compromised everything by introducing emission rules…:slight_smile:

As you say; the vacuum advance, as used by ‘European’ engines is easier to manipulate for best performance - though both ways will run the engine…

xj6 85 Sov Europe (UK/NZ)

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It’s convoluted in that it works the opposite of the ported vacuum ports on a vacuum advance system. Typically a vacuum advance will be connected to a port that sees vacuum everywhere except idle; at idle it provides no vacuum. In the vacuum retard scheme, typically a port is used that provides vacuum at idle and nowhere else.

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Thank you all.
So, it sounds like, the way to go in my case, is “full European”, i.e.

1-choose the distributor with vacuum advance,
2- connect it to a port that does not operate at idle.

1 is easy, the item is sitting on the bench, (although it is the one with mechanical breaker points, the other has an optical setup which I assumed to be superior).

2 gets me perplexed: the only ports I am aware of are on the manifold. Any guidance here?

Thank you.

2 would have to be right at the throttle bodies.
My vacuum advance distributor is hooked up to manifold vacuum and runs fine :thinking: Injected European…

Neither , purchase a 1 2 3 and throw that junk away!
By the mid 70s your car in the states is beaten down between double butterflies , canister filters , detuning strombergs
Put the 123 in and get your car driving like it should
The old junk is junk
And owned all those xj”s😀

No way.

We just did a 2.000+km trip through northern Europe with the original distributor and no problems althogh we faced snow in Sweden and lots of rain in Denmark.



Henry Ford hired the Mallory company to design the distributor for his new V8 in 32. A very nice unit, even if was difficult to service. Vacumn retard principle. Timing fixed, more or less. Off set dog connection distributor to the nose of the cam. A slot and a screw for minor trim. Used til 48.

I vote for the advance unit…


Thank you David, but does not help me since my car has 2 HIF7 carbs… :slight_smile:

Thank you Joey, but the engine I have is a UK specs, with 2 HIF7s … :slight_smile:
123 is attractive, need a budget extension from my side :slight_smile: .

I know, but you can still check around the throttle plates and maybe you’re lucky
But: I bet you can make it work with manifold vacuum? Just tee it in somewhere? Worst case, drill at the right spot…

Eric does the original distributor have only a retard connection, or both advance and retard ? The HS8s that preceded the HIF carbs have the retard connection under the front carb and the advance on the top, if you have both connections the retard needs to be blocked. If you need to drill for a advance connection the hole needs to correspond exactly with the ede of the butterfly, with the throttle closed NOT anywhere in the manifold !!
At idle a retard connection is subject to maximum depression while a advance connection is not.
As an aside our poster in France @Jean-Nicolas had a ignition advance problem at idle, with his XK120 equiped with HS8s, I suspect the advance pipe was connected to the retard port under the front carb, not the advance connection on top,
As allways @Phil.Dobson I might be wrong :face_with_hand_over_mouth:



don’t worry about the distributor. US engines may “suffer” from the many add-ons that Joey describes, but the distributor is probably the most innocent of them, performance-wise. Throwing in a 1-2-3 ignition alone - or any other electronic ignition for that purpose - won’t change a thing, as Pekka mentioned. Doing away with the air recirculation and fitting 2" SU carbs will take you a long way. Finding a MOD gearbox will unleash another 40 horses, I’d guess. So, once you’re at it …

With the twin HIF carbs and the original distributor you just connect everything as per factory - maybe we have someone with a SII HIF around to take a pic. I’d have to make yo wait for the week end.

Good luck


75 XJ6L 4.2 auto (UK spec)

There is a tiny port on top of the rear carb, manifold side. Would that work?

First of all, Eric; all ‘advance’ set-ups require manifold vacuum! Which is 18" Hg (or thereabouts) in idle. ‘Ported’ vacuum has 0 vacuum with throttle closed/idle. This is the easiest way to verify if a spigot has manifold or ported vacuum.

It’s peculiar that several listers confuse the issue; there are two types of vacuum - ‘manifold’ and ‘ported’. There are two types of vacuum regulation - ‘advance’ and ‘retard’. And ‘advance’ use manifold vacuum - as the ‘European’ set-up does…

As Peter says; some carbs have spigots for both - but it is necessary to test idle vacuum to verify which is which! And as David say; you can always ‘T’ a distributor to a vacuum connection with the appropriate vacuum…:slight_smile:

‘All’ distributors has a vacuum regulator - so fitting different distributors still require finding the appropriate vacuum connection.

xj6 85 Sov Europe (UK/NZ)

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I’m not sure about HP as I haven’t taken that car to a dyno, but it would relatively lightly pull to that 110+mph (178km/h GPS) that photo was taken in Essel on the A7 heading south. But on the hills around Kassel I had to occasionally floor it to maintain 120km/h+ going uphill.

Even at those speeds range was 600km on the two tanks, running almost the whole time on overdrive (except some longish road work areas at 60km/h).

The biggest challenge however was lunch as in Germany only take away is/was allowed. The car remains there now, we flew back home yesterday morning.


I connected the advance to your XJ6 manifold (on the bottom, in the middle of both carbs there is a little output where I adapted the connection). Same connection existing also in the front of the manifold, that I am always kept blocked. I have closed every other pipe or hole on the carbs.
Currently I was running with advance disconnected and every hole on the manifold blocked. Planing to bring back the vacuum advance later.
My issue was related to the micrometric screw on the distribution that was advancing the timing much more than what I thought when playing with it.

At last a face to connect to the voice Jean-Nicolas (video)
The connection in the middle of the manifold is NOT for the vacuum unit, at idle the ignition will be way to advanced. The correct connection is as shown , either front or rear carb whichever has the small pipe.0