Im no expert on the matter but you may find that half of your compression issues are simply sticky valves and rings given the amount of time it has sat. If you want to get it running before a rebuild ide personally pour a teaspoon of oil down each plug hole and crank it over a bit. Ive heard some people recommend diesel for stuck rings. I doubt the figures were that bad when it was parked as it would certainly not have been drivable. The only way that it would have lost that much compression while running is if someone cooked it severely.

Old honda mower engines were notorious for loosing compression through stuck valves when they sat over winter, if you pulled them over for long enough they would suddenly free and away the they would go.
Maybe pull the cam covers off and pour a mixture of 50% kerro and 50% motor oil down each 'bucket' and let it sit for a few days, it might be thin enough to soak down and free the valves of they are a bit sticky. It is also probably valves if the compression does not improve a fair bit when you tip some oil down the bores.

I have a friend who shaved a series 3 xj6 head down to nothing (the series 3 have bigger valves), stuck a set of triple su's off an 420g and did up a model 12 borgwarner, stuck the whole lot in a series 2 and the thing is pretty mental for an old jag. Loads of torque and goes like snot. Only problem is with such a high compression ratio the thing diesels like a dog when hot, easy solution is stopping it in drive but its still not ideal. I know someone else who did very similar things in a series 1 and is also very happy with it, to the extent that its now his daily driver.

It all depends if you want something a bit more old school or whether you want something actually fast. If you are a purist but not to fond of the xk you could look at an aj6 conversion as I am at the moment.....

Regards,
Jay

 

I don't know why I forgot about shaving the cylinder head to raise compression. That would certainly be a cheaper option. I'm at altitude so 11:1 compression is pump gas friendly.

As to the old vs modern issue, I've thought about a xjr 6 supercharged, but a high compression, na motor just sings to me.

 

So I have both a V12 and a straight six. Stick with what you have. The V12 is nothing but a huge headache. Very finicky to keep running.

Sticky valves will rob compression obviously. On my straight six, years ago, all I did is take the head out, had it rebuilt and put it back on the block. It served me well for 100K miles.

This build will cost you a bunch of money and time *if* you stick with it. Trust me. I would try to avoid entertaining any enhancements at this stage. You are on the right track in trying to get her running as best as you can on what she came with.

 

It's funny. I started out just like you. Got me a series 1 which had been doing nothing for 10 years.

I hoped I would get it running. After many problems, I took of cilinder head and found severe scratches in 2 of the 6 cilinder walls, leading to poor compression. Even then, I tried reparing it. I took out cilinder lining, but only then I realised the costs on parts alone would exceed the costs for an xj40 scrapper.

And now I got an aj6 in my series 1. :-) And loving it!

 

Well today I started tearing into some of the electrical system, cleaning components, greasing up connectors, and re-assembling. I've found a few wires that are disconnected so I need to sit down with the schematic and figure out where they should be going...

In addition to the electrical components, I've started stripping down the interior starting with the door panels (works with the cleaning electrical components), and I decided to try and clean up the valve covers since I had them off to inspect the valve-train and camshafts. They turned out ok, but I'll need to give them another round.