Jaguar Repair: Rough Idle, intake leaks, vacuum test

Question
I have an Australian delivered 1986 XJ12. Regardless of running 91 or 98 octane fuel it still idles roughly as if it is running on 11 cylinders, once underway it seems be run smoothly although it dosen't seem to have the go in it like it used to. I am currently using ordinary unleaded (91), should I change to the higher octane?
Plugs were changed 3000 klm. ago and mileage is now is 150,000 kl.

Answer
Hi Bruce,
The octane of the fuel is not going to correct a miss or rough idle in an engine. You need to have some tests run on the engine. Unless you have the correct tools and equipment, this is not a job for a DIY. You should have the car put on a scope to test the ignition system and pull a compression test, a vacuum test and test for intake leaks. If the car is injected (I have no info on an Australian version of the XJ-12) the injectors need to be tested and cleaned.
Howard