So I've decided to disable the 15-minute timer and remove the related vacuum lines, valves etc. After comparing the way the vac lines are connected in my car to the diagrams, I can tell things are definitely not in the original state right now.

I managed to figure out most of the circuit, but one part is still giving me trouble, which is shown in the picture. The line I'm holding goes straight to the vacuum advance unit on the distributor. As you can see, it's connected via a T to a vacuum line on the fuel rail, which the white striped line connects to the fuel pressure regulator. Anyway, from what I could find, it seems to me that the vacuum advance line has no business of being connected here.

The question is, is some other line supposed to be connected to that T or was that added as well and I can just connect the line coming in from the right directly to the fuel rail?

 

I agree.

Looks like the T was added, yes, as it isn't the style used by Jaguar



You can eliminate the T entirely and use a different source for the vacuum advance.

The device on the fuel rail is a fuel temperature switch. It operates by dumping the vacuum signal to atmosphere.

Also odd is that your engine has been fitted with an early style fuel rail, late 70s-early 80s vintage.

Cheers
DD

 

Thanks a lot Doug.

The fuel rail has also been confusing me for a while now, glad to have it confirmed. As it's not in the best shape anyway, I might try to source the correct one and upgrade to it when I pull the inlet manifolds to change the leaky valve cover gaskets.

 

Finally got around to cleaning up the vacuum lines. Night and day difference, the car actually holds steady RPMs at idle for the first time since I got it! So satisfying to make it run better and clean up the engine bay at the same time. The idle still drops down a little though. It's not like before when it would fly between 1500 RPM and nearly stalling, now it was around 900 cold, with an occasional drop. Don't know what to look for, did new spark plugs (properly gapped of course), spark plug wires and checked cap and rotor on the distributor when I had it out of the car to fix the seized centrifugal advance. I also cleaned up the resistor pack connector and built a brand new injector harness. The car's been sitting for a long time so maybe it would just clear up after a while?