Well, here goes.
I started it on a Google Drive document, temporarily cut and pasted to here. That document can be shared and edited by other parties, very important since my knowledge is limited. I have a 1979 coupe bought in 1984 which I ran for 10 years stock standard before modifying with a Motec aftermarket EFI system.
Memories of the pre HE system are now hazy. My other car is a 1988 Lucas, so I have no hands on experience of Marelli cars, nor any experience with facelift and 6L cars.
For this reason I hope those with more knowledge will help to add and edit the check list.
Kirbert’s book on the XJS is well known by us, but perhaps not by those who have a rush of blood to the head and buy a non running XJS purely on a whim. Bad choice of course, but we all suffer from whims at times. The “Book” is around 700 pages and has just about everything there is to know about the XJS. It has an index and page numbers so the plan is to refer to that information as a supplement to the check list on a “needs only” basis. Ideally anybody even remotely thinking of buying an XJS, running or not running, would consult the book first. It does take a while to get through the Book and absorb the details.
Chances are that does not happen hence the desperate pleas for help we can find on this forum.
The check list is for the first step: You buy a non running V12 XJS. You at least want to get the engine cranking and firing to make sure it is viable. Later on you work your way through the many possible big or little gremlins to get the whole car shipshape. That is when you should read the Book, all of it.
The cut and paste is below. You will note the information is not 100% accurate at this stage.
I have concentrated more on the format. The check items 
I think should be 2 to 3 pages long.
It has to be manageable by somebody who might be in a hurry and has a lot of previous experience fixing engines. That person will have little need to chase up supplementary information.
Others will welcome copious references to the Book, diagrams and photos as they work through the list.
Nothing is set in stone. Feel free to comment and suggest. Next stage is to share the Google Drive document for more work.
DON’T: Probably a bit late, but hopefully you read this before you buy a non running XJS. They come in various conditions.
a) Neat looking car, good body and interior, just one day the engine stopped and owner or mechanic cannot fix it. Could be a simple fix for a DIY or a mechanic with V12 experience and this forum to help.
b) Ditto, but this time it is serious. Overheated and dropped valve seat, engine bay fire and so on. Fixable for sure by DIY at moderate cost but expensive if you pay a mechanic.
c) Crappy looking car, not well cared for, strictly for a masochist. Just don’t, even for a DIY can be a money pit.
NOTE WELL: There is a decision point about when to keep going with time and money, or when to give up. Firstly the general condition of the body and interior makes a big difference to that point.
If they are both good it is worth going further to fix up the engine. Worst case an engine can always be rebuilt or a second hand one found and you will end up with a car you will like.
If the body and interior are poor you might not want to sink a lot of time and money into it. Better to save that money plus some more and buy a better car.
V12 CHECK LIST: The idea is to at least get the engine cranking and firing.
This is mostly intended for a new owner dealing with a non running V12 car. The idea is to check the known weak points before trying to start the car. Temporary fixes can be made and once the engine is running reasonably well permanent repairs/modifications are advised.
This check list can avoid damage, frustration, wasted effort and buying unnecessary parts.
The check list follows with items to check marked by a . Tick it off when checked.
Where there is more detailed information and photos at the end of the list it is noted as (#1) etc.
There is a downloadable “Book” created by Kirbert which has many pages of information on the XJS.
Where it is helpful reference will be made to this book for detailed information.
The engine bay is probably dirty. Cleaning it up so all the wiring and components are easily visible is time well spent. Especially clean out the valley. (#1)
Check the oil and coolant levels, they should be reasonable. If the oil looks dirty and suspect best change it before it can cause more damage. At this stage spend the minimum until it looks like the engine will crank and be worth persevering with.
Identify which version of 5.3L V12 you have: Pre HE. HE Lucas ignition. HE Marelli ignition.
Later face lift cars are 6.0L. (#2)
Get familiar with all the components needed to make the engine run. See diagrams (#3)
To get the engine ready for cranking check the following items:
There are a number of ground points in the wiring. Black wires are from the negative ( ground ) side of the battery and must be checked/cleaned where they ground to the chassis or engine.
There is a flexible ground strap from engine to chassis close to the oil filter, very important.
There are lots of plugs and sockets in the engine bay wiring. Not essential right now but a good idea to pull them apart, spray with something like WD40 ( there are better sprays for this ) and reconnect. Note the white rubber bullet connectors favoured by Lucas can be very hard to pull apart but there are few reports of these being a problem. The black plugs on the injectors and other sensors are more likely a problem.
Remove spark plugs and check condition. Pre HE should have 35thou gap, HE a 25thou gap.
Verify the firing order of the ignition leads.(#4). Check they have good connections to cap, plugs and coil. If in poor condition they should allow the engine to fire, but perhaps not on all cylinders.
The HE ignition amplifier is mounted on the B bank inlet manifold. They can fail but are easily fixed. There is a co-ax or shielded white wire from amplifier to ECU to initiate injector pulses. Nearly always shorts to the shield and needs fixing. (#5)
Lucas ignition section: ( Skip if you have Marelli )
Remove the dizzy cap. Check it for any damage and a functional carbon brush.
Later cars have a nipple in the cap for connecting a breather hose.
The spring loaded advance mechanism can be stuck, it should rotate freely. A bit of oil helps.
The springs in the mechanism may stretch, will not stop engine running but will affect timing.
Check the condition of the vacuum capsule. (#6)
Pre HE has a bracket with 2 reed switches to initiate injector pulses, sometimes replaced with solid state switches. The pre HE ignition plck up uses a plastic wheel with 12 small bar magnets. Check the gap between magnets and the coil, should be ? thou.
Pre HE used Lucas ignition amplifier mounted in the valley, later above the radiator.
Known to fail, often replaced with aftermarket types. One update was Crane Cams amplifier kit which included a 12 slot wheel and optical switch for the dizzy.
The HE pick up uses a 12 lobe star wheel. Check gap between lobe and coil, should be ? thou.
Marelli ignition section:
Verify the firing order of the ignition leads.
Marelli cars have sensors ( pick up coils ) on the front of the crankshaft and the flywheel.
Check they have the specified gap ?. They are prone to fail. Check the resistance which should be ? ohm.
Fuel injection section:
Injector wiring in the valley. Often in poor condition with broken insulation and exposed copper. Temporary fix is to cover the copper with insulation tape. Avoid shorts across wires or to engine.
Later rebuild the harness or buy a new one.
Injector fuel hoses are known to fail and cause engine fires. Check carefully and if there is any doubt replace them. Once the engine is running replace them anyway unless obviously fine.
Various items sending signals to the ECU that need checking.
A vacuum hose from the middle of the rear crossover tube to the ECU.
The coolant temperature sensor should be about ?ohm. Critical to start the engine.
Air temperature sensor also ?ohm, but not so critical.
Throttle position switch(TPS). Can be a problem, see (#7)
The throttle movement on both sides should be checked. The rubber bushes in the linkage often crumble, does not stop the engine but can cause rough running.
In the trunk there is a small tank under the battery. This is almost sure to be full of crap and a plugged up filter. A constant source of trouble, remove and clean.
Other things:
Check the belts on the front of the engine. Will not prevent cranking but if slack or missing the engine gets overheated. The fan has a viscous clutch, rotate by hand and you should feel resistance.
Check battery condition and connections to battery terminals.
Check ground cable from battery to the bolt on the chassis. Use trickle charger.
If the car has not been running for more than a few months chances are the fuel will be compromised. Drain and refill with high octane fuel.
