XJS Misfire...
 

Well I changed the oil and got the transmission sump repaired, but when I fired it up it had an AWFUL misfire! I changed the spark plugs, HT leads, distributor and still nothing. Just won't run as it did 4 days ago! Please help me! I am starting to think about low compression now, could it be the piston rings or valve seats ? Are these common to go in a 4.0 ? It's weird if it is low compression as the engine only has 9,100 miles on it!

I don't know what year your car is, but my 85 xjs was stored in a garage for a number of years and i've found a bunch of the vacuum lines were rotted out (probably from all the heat these put out). Maybe some of the lines got moved around and came loose or tore when the car was being worked on. I'm trying to track down a similar problem, but fixing the vac lines didn't fix it.

Its ok now. Just a bad spark plug cable!