Vacuum Regulator Advance Experimenting

HE V12 with Lucas Ignition

did some timing readings last week at idle with a specific inHG vacuum applied to the Vacuum Advance. These are at idle with timing set at 18BTDC @3000RPM:

0 inHG = 2 BTDC
4 inHG = 4 BTDC
5 inHG = 6 BTDC
6 inHG = 8 BTDC
7 inHG = 10 BTDC
8 inHG = 12 BTDC
9 inHG = 14 BTDC
10 inHG = 16 BTDC

The brown vacuum regulator supplied with my 1988 V12 XJ-S is 10 inHG. But I like my base timing set to 22 BTDC @3000RPM (I have raised required octane from 87 to 93). So this gives me 20 BTDC at idle, which causes a slightly irregular idle sometimes, and seems quite advanced to me for an idle. (I also run 180F thermostats, instead of original 190F, so my idle is probably a wee richer than stock)

There are many GM Vacuum Regulators from the 80s, with a color code. I have no idea what color means what, so I’m buying one at a time and trying it. You can sometimes find these NOS for a decent price of under $20.

White/Yellow GM 14006719
Green GM 14006771
Pink GM 14015597
Black GM 10115851
Grey ???

I am now running the white one, which is also the same one that the XJ6 uses. It supplies 4 inHG to vacuum advance. So my idle timing is now at 8 BTDC instead of 20 BTDC. I do get a better idle, but I’d like it advanced a bit more. (My V12 seems to idle the best at 10-12 BTDC) I have purchased the Green one, and will test it next weekend.

If anybody knows how the color corresponds to inHG, I would love to know! Otherwise, I’ll keep posting my findings.

For those of us who want to lower the advancement at idle a tiny bit without retarding the base timing, there seems to be options. And I don’t like running the vacuum advance off throttle tap, because it’s just too low of an advance at idle for my liking.


Installed a Green one today, GM14006771.
It controls idle with 5 inHG to vacuum advance.

White one GM14006719 controlled idle with 4 inHG.
Brown one (OEM) controlled idle with 10 inHG.

I won’t be buying anymore, because the green one works perfect for me, giving me 10BTDC at idle running a base timing of 22 BTDC@3000RPM.


Thank you for your Vacuum Regulator investigation and for posting your findings. I am certain that this info will be helpful to others.


Thanks Paul, I really enjoy fine-tuning this engine, glad to know that i can contribute to the community.

The brown OEM one seems to be Jag specific, and pretty much unobtainable.

The green one is out there on ebay as NOS, quite a lot available as it was used for many early 80s GM V8s. For emissions on carb engines.


Greg this is fantastic info! The brown one on my '89 is a pretty crispy critter and wasn’t sure what I was going to use to replace it.



I also relocated mine so it stays cooler. Original spot above exhaust manifold is not the best place!

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I updated info above, after a few days running, the green one provides 5 inHg. (Not 6) So maybe it took a while to settle?

Still a good idle timing for me with my base +4, but someone running stock timing would only get 6 BTDC, maybe not an ideal idle.

Want to add, I’ve noticed the idle when cold (up to about 160F) is a bit rougher/lower RPM than it was with OEM vacuum regulator. It idles excellent once warmed up.

I assume that’s because the AAV is not open enough, assuming a more advanced idle. Once the car warms up, the extra opening of the AAV idle screw supplies enough RPM. The idle screw does nothing when cold, correct?

maybe I need to open the throttle clearances a bit to help with cold, and adjust the aav idle screw closed a bit to compensate for warm?

Minimal, but it adds to the total amount of air entering the engine.

Opening the throttle clearances will have the same effect as opening the idle screw on the AAV, i.e. will have the same effect on both cold and warm conditions.

The best solution would be to modify the port on the AAV to give more air when cold.

A thing I did was to add a thermostat and control the Extra Air Valve to open under a certain temperature and boost the idle.

Also, when I rebuilt my AAV I’ve added a screw at the piston so I could fine-tune it’s position when cold.

Cylinder_02 Cylinder_01 Cylinder_03 .

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What does base timing at 3000 mean? Are you saying your full mechanically advanced timing is only 22 degrees?

Yes. My USA model specifies 18 BTDC @3000RPM, full mechanical advance. I set mine 4 degrees more advanced, and upped my octane by 5 points. I made sure to set it at 3000RPM. It so happens the idle advanced 4 degrees too. Which is why I swapped vacuum regulators to lower that advance at idle.

I may remove my AAV and test it. I got a rebuilt, not sure if it’s working perfectly when cold.
Modify the port? Do you mean simply filing out a bigger hole? Won’t that cause the piston to never close?

I think if the top of the AAV is pressed in too far, it reduces the port size, which in turn reduces the cold idle. John John did yours if I recall, so I would assume it is correct. There is a measurement to verify; google Jaguar AAV overhaul.

The V12 H.E. gets more of its advance from vacuum and less from centrifugal than the pre-H.E. That said, note that 3000 is not full mechanical advance, it will pull farther at higher RPM. It’s set at 3000 because that’s near the peak torque and hence the point where correct timing is most critical.

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As always, you are correct! Idle RPM at cold is exactly the same. So I set throttle stops back to my .002" and opened aav idle screw back to where it was. (Always take notes! I also mark things with finger nail polish!)

I may be tackling this from the wrong angle, assuming more RPM will help. With this 10 BTDC at idle instead of 20 degrees, I’m getting a slightly better idle warmed up, but worse when cold. It could just be that the 1.9V setting on ECU fuel mixture is intended for a leaner mixture with a more advanced idle. So all I need to do is enrichen 2 clicks on the ECU? I will try that next.

I’m starting to think the ECU fuel mixture setting and higher than normal advancement at idle was solely for a leaner mixture both for emissions and fuel economy in the US market?

You don’t change the height of the hole but the width, so you just get some more air at the cold position, but the piston will still close the hole when warm.

You can do a test, unhook the vacuum hose from the extra air valve, so it’s open, and see if the higher RPM will make a difference.

Could be.
But as soon as the ECU goes in to closed loop the fuelling will be according to the info from the O² sensors and not the base fuel map, so I I think it will be the same as before.
Would be curious to see if it makes a difference.

Isn’t it strobed with vac plugged? I can’t remember how I checked my several V12s although I never checked either of the 6.0L with or without distributor. The whole point of strobing for max advance (to me at least) is that you avoid high speed detonation under load by accounting for any slop or worn slots or stop pins in the centrifugal mechanism. By timing at lower revs on a part mechanical system, how can you be certain that it doesn’t over advance and the spark is optimal when it matters for engine health, not just transient pinking?

It has to make some difference, or else why is there an adjustment for leaning/enriching idle mixture for closed loop? Its not changing base map.

No, idle mixture and base fuel map are the same thing.
It’s the base for fuelling when in open loop and this value will be trimmed to either direction as soon as the ECU goes on closed loop.
The ECU is on open loop bellow a temp threshold when cold, on idle (therefore idle mixture) and WOT.
The 1.9v adjustment is basically using the O² sensors as a guide to set the base map at the middle of the trim, i.e. stochiometric.

Yes. That’s why it’s normally only 18 degrees, or 22 for Greg because he’s gone 4 degrees past spec.

The point you have correct. What is incorrect is the “max advance” part. You’re not timing at max advance, you’re timing at max torque, which is actually closer to the “knee” on the centrifugal advance map.