A gremlin hiding in your trunk..
 

That cute little surge tank in your trunk may be creating the next major problem you will have to face. Inside there is a plastic screen that keeps large particles from getting to your fuel pump...that is, if it is good and still in place on the end of the pickup tube.
When I first purchased my XJS, it had several issues. One was that the plug wires were on the distributor in reverse order and even out of order. The car ran, miraculously...and I limped home with little to no power. The first thing I did was to make sure I had a clean fuel supply and the right pressure on the fuel rail at the engine. I pulled the surge tank and this is what I found in it.
Attachment 167797
The sludge is non magnetic and that means it is not rust flakes. The upper tank is shiny and clean and after I cleaned out the sludge, the surge tank was too. The screen filter was completely plugged. What is it?...I have no idea.

I read somewhere..that a good idea was to remove the screen filter altogether and install a real filter outside the surge tank..before the fuel pump. My car was also missing the fuel filter after the pump. After a few months of trying this, I found that I was getting a whine from my fuel pump..I suspect that the pump is cavitating trying to suck the fuel through the filter I added in front of it. The pump is not made to do this. It is better at pushing than pulling. (I am in the process of replacing the screen filter in the tank and installing a new fuel filter after the fuel pump...as it was originally designed. Hopefully this will resolve my pump whine..if not, I may have a leakdown situation.) I would recommend that everyone check the screen filter and sludge/sediment situation in the surge tank. Being a lower tank, that is where everything will end up. This is not something you want to get into your fuel pump or injectors!

its gasoline residue, IIRC from evaportation. have you ever seen mad max (road warrior)? remember the part where he wrecks the max mobile at the end and all that red dust goes everywhere?

poor quality gasoline. just run 93 octane, a few more degrees of timing, and youll be all set, at least for 25 more years.
plus more timing = more horsepower and a cleaner burn.

Am I correct that the later model XJS did not have a surge tank?

Thank you for reminding me of this little bitch, it was on my todo list and I forgot. I can only imagine what I have in it, since 1982, plus my car was owned by some hillbilly from London (Ohio, not UK), I can only imagine what kind of farm grade fuel he was using in it. BTW, I already placed extra fuel filter, upstream from pump, to make sure.

I believe the surge tanks are pre 1992.

A clogged pickup screen and a dirty surge tank can lead to major problems. Very detailed information can be found in "Experience in a book" as to what symptoms and failures can be expected when it does clog. The most simple being a new fuel pump and the worst..an engine to rebuild after running too lean.
I believe I may have saved my engine by doing the maintenance when I did. It was running very poorly when I bought the car.

Here is what I got:)
Pickup screen was busted inward, due to clogging, so at least it did not starve the motor with fuel. Amount of sludge was lesser than yours, however definitely sizable. Sea Foam busted sludge off the walls in few minutes and I dumped the rest in fuel tank, after letting fuel pump suck in about 1 qt of it in fuel system. I even cranked the motor to let it squirt Sea Foam in cylinders and such, so now it will work its magic on my car's giblets. See some pics.

Looks like you got it squared away. Even a little bit of that crud is too much. I have ran injector cleaner through mine a couple of times and I too am a fan of Seafoam products. I think I'll do the same.
Nice paint job!

What grade filter did you use before the pump?

You need a 100micron before and a 10micron after the pump. If the filter you installed before the pump was too fine this will be the cause of your cavitation.

I am assuming that I installed a 10micron before since that is the standard for gasoline filters. I bought it at NAPA using the only requirement to be 1/2" barbs on both ends. The fitment is unknown since it probably would fit several applications.
I have a new OEM strainer for the surge tank in hand and the proper upstream filter is available over the counter and most all auto parts stores. I am waiting until my tank is low on gas and I am in the mood to put it back to the original setup. It lasted many years before building up the crud so I assume it will outlive me.
The whine may be from this but it could also be a pump going bad. I didn't replace it after the surge tank clean out since it was still working and tested good. The fact that the whine only appears after sitting a day or two and goes away in about 15 minutes of running..suggests to me that there may be a bad check valve in the pump.
Thanks for the reply.

Hmm this is just the cause of my issue it was running just brought it too dells an back not more than a 2 miles trip got on her a bit it popped stumbled an shut off. It sat for 9 years ...definitely going too tackle this on my day off