About six months ago I went out to start my XJS it fired straight away but was rough as it appears to be running on only six cylinders, any touch of the throttle causes it to cut out.
Any suggestion as to what could be causing this problem.

 

What year is the car?

 

This is important as there are 3 different ignition systems used on the XJS V12 over its life.

It is a good idea to add the model details to your signature as there were many upgrades that Jaguar did over the life of the car.

 

Hi There my XJS HE is an '85 model with a Lucal Distributor and I am told Digital P fuel injection. Thanks for the responses

 

I would tackle it in this order

Pull all of the spark plugs out and if they haven't been replaced in a couple of years put new ones in. If they are ok clean and check gaps and re-install. My car was laid up for 2 years and plugs were new before I took off the road but it still only ran on 9 cylinders - just needed the plugs cleaning, good as new now.

Sticking injectors would be next on my list - not quite so easy to find. You can use a 9Volt battery, disconnect the injector electrical plug and carefully touch the wires on the injector terminals OR better would be to buy one injector connector, the injector should click. Be careful not to short the battery wires or it will spark.

Start here and see how you go.