Distributor vacuum regulator

Hey guys. Can anyone help me figure out what the replacement part number would be for the distributor vacuum regulator on a 1986 xj6? The number on I says EAC-4399. Mine is bad and I can’t find a replacement anywhere!

Cheers
DD

Thanks for the link. He doesn’t list one on his site that I can find. I’ve looked at all of the usuals like jag bits and everyday xj but can’t find the darn thing…

Call him. He parts out lots of Jags. it would be hard to believe that he doesn’t have one

Cheers
DD

My Lucas equipped V12 uses EAC 4012. Wonder if that would work for you? Or replumb the system and live without it!

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How do you know it is bad…?

As Dave says, you can likely live without it - earlier xj managed without it, but it depends a bit on the vacuum advance set-up…

Frank
xj6 85 Sov Europe (UK/NZ)
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The diaphragm is torn inside. Took it apart when I found no difference in vac on any of the outlets. When I bypass it I get a backfire through the intake. I have a Petronix dizzzy and have no ignition amp any more.

I’ve replaced all fuel injector rubber, sent the injectors out to be cleaned and matched, replaced all vacuum lines in the engine bay, and replaced all rubber parts in the intake as well as Cruise control. Still can’t get rid of the rough idle. I’ve set the timing to 17 btc and checked the vac advance is working with a vac pump and gauge.
There are 2 vac outlets under my intake manifold that are caped off. I see the one is a drain on all of the diagrams I’ve found but can’t seem to make sense of the second. Any ideas?

How rough is ‘rough’?

If you’re expecting glass smooth, well, you have an uphill battle ahead. Very few, if any, others have have reigned victorious. A slight tremble is typical.

Cheers
DD

What is this thing supposed to do? I use that dizzy, although with carbs. So I’m curious.

Take a look at - jag-lovers.org/xjlovers/info/XJ6vacuum pdf. Maybe that will help. The link may not work, so type it in!

This link may work

http://www.jag-lovers.org/xjlovers/xjfaq/delayvalve.htm

Short story is that it’s a vacuum delay valve to control vacuum to the vacuum advance capsule.

Cheers
DD

The vacuum controller is not a delay valve but rather a controller that supplies a steady minimum amount of vacuum at idle. Above idle, it lets ported vacuum take over when the port is above 6 in of vacuum for the XJ6 and 11 in for the V12 HE. At full throttle, there is not much vacuum so it drops off. This extra advance is to give the engine more advance ignition timing at idle to run cooler and use less fuel. This unit is also used on some American cars as one of the ports is labeled “Delay”. This unit does not delay vacuum but that is where it is hooked up on other non-Jaguar applications. Easy way to check is to measure the vacuum at the distributor with the above mentioned specs assuming it is hooked up properly. Delay=manifold vacuum , Dist=distributor vacuum unit and Carb=Ported throttle (not direct manifold vacuum)

2 Likes

Thanks, Dick !

Cheers
DD

Dick, I won’t even bother to ask how it does what it does. By ported vacuum I assume you refer to the throttle body?

But to the brass tacks…using this dizzy with triple SUs, am I better off connecting it to the ported vacuum from the front carby, or to the manifold directly, or to leave it disconnected, or other (perhaps incorporating this regulator)?

This regulator helps when you need more timing advance at idle but don’t need the advance under load. Such is the case with the V12 HE. The 11" of vacuum increases the base idle timing from TDC to 20 BTDC at closed throttle. The early HE’s had a vacuum delay switch to disable the vacuum to the distributor for 45 seconds. This lowered the idle so much that there was a electric solenoid air supply in the right side air filter to step the idle back up during the 45 seconds. Setting the base timing at 20 BTDC and using ported vacuum only would result in severe pinging and to much overall mechanical timing. This effect does not make as much of a difference on the XJ6 but then it does not change the timing but a few degrees. In answer to your question, with the SU carbs, it is best to just use ported vacuum off of the front carb. Light throttle cruising does better with the vacuum port working with the distributor.

1 Like

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If you have a Petronix dizzy, you must(!) refer to the Petronix manual for the set-up. Petronix works differently from the Jaguar set-up…

Don’t know if the Petronix use a vacuum input, and what input - but unless specified in the Petronix manual; a vacuum regulator is not used. The two types of vacuum source can be identified with a vacuum gauge at respective spigots. Manifold vacuum, spigot usually on the manifold, shows 18" Hg in idle and varies with engine load. Ported vacuum is connected to a spigot on the carb(s), and usually shows zero vacuum (or thereabouts) in idle - varying with rpms and pedal position.

It’s important that the proper vacuum source is identified - the source generally defines initial advance setting.

Quick-check; set ign timing to 4 deg BTDC in idle with vacuum disconnected - and see what happens. Backfiring indicates too much advance for the Petronix - but consult the manual…

As an aside; if you set 17 deg BTDC and add manifold vacuum - the advance will be far too high, and the idle likely very bad…

Bearing also in mind that uneven idle has many interpretations, as Doug says - have you checked compression of late…?

Frank
xj6 85 Sov Europe (UK/NZ)
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Thank you all!!! I will get back in it this weekend and see if I can get it to run better now that I have a better idea of what I’m looking at. With all of the work I’ve put into tuning up the old cat… it was disappointing when it ran so rough the first time I started it.

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As Doug said previously - if you want a silky smooth idle it’s an uphill battle.

Idle rpms influence smoothness - as rpms increase the engine smoothes out and is ‘silky’ at higher revs even with serious issues. So your idle rpms is not without interest…

The inlet backfire may disapper with proper advance setting - but also; have you carefully verified ignition sequence? If plug wires have been disconnected it is quickly done to misplace them - which will make the idle very bad indeed…

And if still uneven; do a compression test to ensure that the engine itself is not the cause…

Frank
xj6 85 Sov Europe (UK/NZ)
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I got a new vacuum regulator and get it installed and Re- set the timing to 17 btc with the vac advance off. The car runs better but it bogged down a lot when trying to accelerate. The idle is steady at 8000 and it sounds good. Revs great when not in gear but stutters and doesn’t want to go when driving…

It may be time to take it to a pro for tuning… Can have the compression tested as well…

Jason,
Have you connected up a fuel pressure gauge and observed the fuel
pressure when the engine bogs down? I like to install the fuel pressure
gauge in place of the cold start injector on the fuel rail, then start up
the engine and use the thottle cable to increase RPMs. Fuel pressure
should be around 36 PSI and stay pretty much constant. If it doesn’t then
you have reason to look further into fuel system components.
I have encountered problems like this before in my Jaguars and it was
due to fuel delivery issues including clogged fuel filters, marginal fuel
pumps or clogged fuel strainers in the fuel tanks. I was able to identify
the problem was fuel delivery by watching the fuel pressure drop as the
engine stumbled. Since encountering these problems I have installed
additional aftermarket fuel filters in the spare tire well between the
tanks and the fuel tank changeover solenoid. I have found badly clogged
fuel filters, debris filled fuel strainers in the fuel tanks and marginally
producing fuel pumps can cause an engine to bog down above idle (I am sure
you meant 800 and not 8000 for your idle).

Paul

Paul