UK Car Repair: Idle Speed on 1984 Jaguar XJ6, inlet manifold design, plug gap

Question
I have an 84 Jag XJ6 that I love, but the idle is a little rough.  I have replaced the fuel filter, which took care of flat spots when accelerating from a stop, but didn't do too much for the idle.  Is it possible to adjust the idle myself?  

Answer
Is it the 4.2 Engine? Idle quality has long been a problem on cars with higher mileage, this is generally considered to be a inlet manifold design problem. There are two adjustments, there is a hexagon adjuster on the front side of the throttle body, it is tricky to get to. Then of course, the idle speed adjuster.  Have you checked plug gap and timing?