Jaguar Repair/Engine Miss fires . '83 4.2.L XJ6
At 2300 rpm the tach starts to "kick down' and you can feel the power jerks. As you speed up to about 2500 rpm the engine runs smooth again. I replaced the Delco thing inside the Ignition Amp. then put a new Coil on. Also looked at plug wires in black of night for odd sparks-- nothing. When it is about 30 C ( hot ) outside this is more prevelant. Today I drove 45 Km and it was very bad at 2300 to 2500 so I stopped in a rest area---- and throttled up to almost 3000 rpm several times------I paused around the mentioned revs-----but no way would the engine faulter while parked. Once driving again the problem continued right at 2300rpm. I does this in all three gears if you use trans lever to hold there. (automatic BW66) The engine pulls good and I can take up to 4500 rpm or more in first and second. Some one suggested that the ground strap from engine to body may be deteriorated and breaking the circuit. Will have to get under the car to see the cable he is referring to. I live in Ontario Canada----- but this XJ6 I brought in from North Carolina USA Just went past 100,000 miles this summer.
The tach reads the ignition pulse so if you get a problem that is associated to the tach jump, there is a good chance that your problem is ignition related. A conformation can be had with just a timing light. Connect a timing light to the COIL wire not a plug wire and run the timing light out and put it under a wiper blade and tape a piece of cardboard over the flash end on the windshield so you can see the flash in broad day light. Drive the car to the point of the miss fire and note if you see any blinks in the steady flash at the point of the miss fire.
If you know a shop that had a scope, that too will tell for sure. I have never seen a tach affect ignition but often seen ignition effect a tach.
Engine grounds can cause many electrical problems so to test that just put a volt meter on a low DC volt scale and put the (+) positive test prob on a clean part of the engine metal and the (-) test prob on the negative POST of the battery (not the cable end). Rev the engine up to about 2000 RPM and read the voltage and it should not be much if any above .5 volts. If it is a lot higher then you do have either a bad ground from engine to frame or frame to battery.
If you still don't find anything have a shop with a scope check the wave form of the alternator as a leaking diod in an alternator can send pulses of AC current into the electrical system that can cause ignition problems.
Let me know,