Hello, looking for suggestions.
We have a series 2 with the Stromberg carbs. I rebuilt them a couple months ago, and the car has been running reasonably well until recently. My wife had complained of a loss of power on a recent drive, and a misfire at idle. My test drive confirmed a misfire at idle. I identified multiple bad plug wires, if unplugged while running they would jump/arc from random parts of the rubber jacket. So, I replaced the plug wires. This evening was the first drive, I drove a half hour out to my in laws with no issues at all. On the way home, my wife is following me, and the car cuts out badly. As if I turned the key off. But sort of intermittent, as if I were turning the ignition on and off. My wife meanwhile witnesses fire blowing out of the tailpipes! I suspect this was an ignition failure, and while āoffā the exhaust loaded up with fuel air mix which was ignited when the detonation came back āonlineā. What on earth could be causing this? The plug wires were definitely bad, and replacing them cured the misfire. I had one perfect drive. And now this? Definitely intermittent. Should I suspect the coil, perhaps damaged by misfiring plug wires? Itās almost time to take her off the road for the year but Iād like to sort this, hate to leave a mystery all winter.
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Watch the tacho when this happens, Jakeā¦
If tacho drops to zero, or seriously deviates from engine rpms as judged by sound/road speed; ignition is intermittently lost. This may have various causes - so what ignition system do you have?
With mechanical points; checking points for wear/gap is the first step. With EI; the tacho is a reliable indicator - but only tells that ignition malfunctions, not why. Faults may be bad connections, failing coil, failing ign amplifier, failing pick-up, failing rotor, cracked/dirty distributor lid etc etc.
The ādark roomā test is relevant; observing illicit flashes from the high tension leads. And other visual inspections may be relevant - as is the āwire shakingā; wriggling wires with the the engine idling. The engine should be unaffected by wire manipulation - if not; there is a tender spot to checkā¦
Frank
xj6 85 Sov Europe (UK/NZ)
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Thanks for the pointers. The wiggling wires is definitely a good idea. I sort of doubt itās a failing coil the more I think about it because once it gets hot a bad coil continues to be hot and misfire. This issue came on abruptly and went away for the rest of the drive.
Sloppy ignition switch?
Carl
As Carl rightly adds, Jake; ign key to be added to suspects - which are numerous as it isā¦
Another curious addition; water dripping from a leaking radiator hose may intermittently cause āwildā firing. And, which happened to me (on another care); the distributor wasnāt fully seated - allowing the dog clutch drive to slip in and out randomly. Complete loss and recovery of ign - took me some time to accidentally find thatā¦
Incidentally; only failed points or ign amp/pick-up will show up on the tacho - all other faults does not interfere with tacho readingsā¦
Frank
xj6 85 Sov Europe
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Since itās been a few days since the last reply I thought I might just jump in and give my 2 cents 
On my ser. 3 v12 HE I had just about the same problem, the tacho just dying and the car going with it. In my case it was a bad ignition amp, which I believe was caused by pulling the plug wires. In my case it appears that the tach was also fried.
Thats about all I have, sorry, Iām not really sure how the i6 ignition works, but just thought Iād chime in with my experience!
Well I feel Iāve left everyone dangling which I feel badly about. I appreciated all of your advice and planned to try them methodically to eliminate the issueā¦ except it never happened again!
This isnāt the first time this has happened to me either, it seems the old girl responds to the threat of tinkering and stops fooling around! At any rate, I do imagine this reprieve wonāt be permanent, something caused the issue the first time. When it comes back Iāll be armed with knowledge.
Jake,
there may be a certain consensus around this place that certain - and in particular electric - faults rectify themselves by more intensive use. So maybe this was the case with your problem. Keep an eye on it and drive your car regularly:-)
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)