I purchased my first Jaguar about 2 months ago, a 1985 XJ12 Sovereign with the Lucas CEI, it hadn’t been used more than an occasional start every year for 10 years until I purchased it.
The car has during my ownership always suffered from a rough and hunting idle, but performed well at higher revs, fuel consumption has been around 15l/100km which I understand is quite normal.
To try and combat any future issues with rough idle, and to catch up on general maintenance I decided to change spark plugs, and to take out the distributor in order to clean it and service the advance weights.
After reinstalling the distributor I went to start the car, first start was easy, but I noticed a very rough idle, something which I had hoped would be solved by setting the timing. When I then revved the car to 3000 I noticed a very strange behaviour with the tach, it would climb to 2500 rpm then immediately drop down to 2000 rpm after which it would climb like normal; when decelerating the engine the tach follows the same pattern, down to 2000 then up to 2500 to continue down. Here is a short video: https://www.youtube.com/watch?v=s1VeIH2dcC4 Keep in mind that I am accelerating at a steady pace. I apologize if the video is unclear.
When I purchased the car one lead from the ignition amp was disconnected, the previous owner reassured me and said that it was an extra lead which wasn’t necessary. I believe that this is the lead from the ignition amp to the tach, it is the white male bullet connector from the Lucas AB-14 amp. I have no idea how the tach is getting a signal if this lead isn’t connected as I understand that the tach source is the ignition.
I am not sure if the tach problem is related to the distributor service as I believe I might have noticed this issue earlier, for about a month the tach has barely moved when shifting from 2nd to 3rd, even though the revs drop noticeably, the shift occurs at around 2000-2500rpm, so the rev range as the issues occurring now.
Hopefully some of you might have experienced a similar behaviour or have some ideas/solutions!