I had to have my 86 XJ6 head rebuilt about 9 months ago. It ran and idled beautifully. Had to get it smogged (Gov Newsome insisted) and it failed 6 or 7 tries as the shop tried all sorts of things. Finally they got it to pass but the idle is very slightly more lumpy.

So I take my first close look at the vacuum tubes since the rebuild. Several things are different now:
The three way vacuum delay valve EAC3349 that connected to the distributor is gone. The vacuum port on the throttle body connects directly to a three way connector which goes to both the manifold vacuum port and the distributor advance port. That vacuum source also goes to the one way valve for the AC system and the hose to the right front wheel well assembly.

I am of mixed feeling about messing with things. It runs better than it has. I ultimately did pass smog. Is having manifold vacuum connected to the throttle vacuum port potentially harmful to the engine? Can anyone guess what the impact that would have on performance or mileage? I know some of you are thinking, just buy a delay unit on ebay, and connect it back up and see what happens. But, sometimes when I've tried to "fix" things, I've made things so much worse. Sometimes things that should be reversible by reconnected tubes up the way they were doesn't work with my Xj6. It's like there's a finely balanced chaotic system and if I perturb the chaos, I'd have better luck trying to unscramble an egg. Am I the only one who experiences this?

So to the old timers here - what would you do?

 

With the distributor connected to manifold vacuum, the timing setting, with vacuum hose disconnected and plugged, should be about 4 degrees. When vacuum is re connected it should read about 17. The timing should be verified before just adding a 3-way delay. If this is done you need to reconnect the vacuum to the port on the throttle body and set the timing at 17, with vacuum hose disconnected and plugged..

 

Thank you James,

Seems obvious when you say it like that. I'll check timing as you instructed before adding anything. Now I'm more curious. I haven't messed with anything yet.

 

If you have the out of limit for your Calif Smog tests HCL, CO etc, it will help to pin point some alternate possibilities.

Normally if you are challenged by CO at idle, then tweaking to pass with the mixture on the Airflow meter will result in rough Idle most of the time unless that car is fully warm. (you can turn air-fuel mixture fitting 1 turn CW to increase fuel for smother idle. you may need to readjust idle RPM slightly.

I would try plunging the line from the manifold to the 3 way fitting, and cap open 3 way connector. Now your throttle body vacuum is to the distributor and to the fuel charcoal filter. This gives you a California idle with no vacuum until throttle plate comes off the stop. You have distributor at 17 degrees which is base line for mechanical advance ( with no vacuum advance), to 33 degrees at 3000 RPM. This may improve idle slightly.

UK Jaguars do not have the 3 way smog delay valve and connect the distributor advance directly to manifold vacuum ... this give better efficiency and drive-ability... With the 3 way valve in the mix, you get a California idle for best emissions, once you open throttle after a time metered leak delay, you get manifold vacuum directly to the distributor for better drive-ability.

Having the manifold and throttle body vacuum to a 3 way fitting confuses me, as it would seem, you would always have manifold vacuum to the distributor and the charcoal canister.
Rgds
David