My Mk9 had its original HA12 coil and from one day to another the engine, once warm after15 min of driving, began to misfire, sometimes with explosions scaring the s*it out of pedestrians nearby. Shifting to N and revving it up to 3500 for a few secs sometimes sort the problem.
So I got a new coil and rotor from Distributor doctor in England. Things have improved, and some drives have been faultless, but yesterday the engine, as always once warmed up, began to lose power through bad ignition. I dont think its a gas/petrol issue.
Might it be the condensator? I have a new one but on the Mk9 it’s a really tricky job to change, so I have waited, hoping the new PA12 coil would cure the problem.
Change the condenser, I had the same symptoms with my old S11, actually blew one of the resonators apart with the unburnt fuel going through the exhaust.
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Condensors often exhibit that kind of behavior: my money’s on a bad one.
You can also mount the condensor outside the distributor, pretty much anywhere in the engine compartment. If you can find an older style Mallory condensor, those things will last forever, if you’re committed to using points and condensor.
IIRC, the capacitance needed is .22 microfarads.
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It turned out that my condensator was of chinese origin, and only lasted about 5 yrs.
Today I changed it for a new one, from Distributor Doctor, and tge engine now runs silky smooth. So perhaps the original HA12 coil from 1961 was still fine, and the redent condensator caused the misfiring and loss of power.
Thelow and tight position of the dizzy in the Mk 9 must make it the worst to fiddle with the dizzy on.
Took 45 min and use of tape and various tools.
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…ditto the XKs, and any other sedan, using p&c.
That’s why I, long ago, I would mount those hateful little things on the inner fender panel and run a wire to the distributor wire. That way, when they failed, which was somewhat frequently, then I could change it easier than having to reach down into that Abyss!
That idea makes me wonder if there are higher quality aftermarket “universal” condensers of more modern design types that may last longer than Chinese Replacement Auto Parts
Auto Condensor constructions doesnt seem to have changed much since the '40s, maybe that design is the most rugged for its operating conditions ?
Back in the day – and I’m talking in the 60s – – Dad found a stash of the old brass-cased Mallory condensers. I think you could drive over them with a tank and they would still work… 
He found ~500 of them for $2.37, or something ridiculous like that. That’s what I used, for years on a lot of cars, and especially Jags because of the difficulty of reaching the condenser inside the distributor, which I always thought was a stupid idea.
I also did the same on DKW’s because to change the condenser on those you had to take the timer off the front of the engine and replace them. I would mount them up by the coils, which were mounted on the cylinder head, and again, it made it easier to change them.
Although, once you put in a Mallory condenser, you could usually count on it for decades!
Any good quality .22 µF condenser, with some kind of an external bracket, will work.
Standard Motor Products LU209 condenser on XK120
A condenser or capacitor is basically two sheets of metal with insulating material between them, and wrapped around each other in a roll, so they are basically parallel to each other and separated by the thin insulator material.
One sheet is connected to the end terminal screw or wire, and the other sheet is connected to the body, which is grounded.
The usual failure mode is for a short circuit to form between the sheets, which may only connect intermittently when the capacitor gets warm, and then separate again when it cools off.
This short circuiting is what causes the poor running or non-running.
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This has been a very useful discussion, and I have learned a lot. My ownership and driving of Jaguars date back to 1985, so it’s never too late to learn more, this time through personal experience, which beats any books or stories.
And it explains why the Mk9 would begin to misfire at about the same geographical point on each drive. About 10-15 min drive following a 5 min warmup at idle.
Thanks to all, and Distributor doctor for providing the best components.
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Hi Peder,
Nope, the LHD MKV has got to be the worst to work on, it needs the expensive side entry dizzy cap as there is no room for the wires under the steering column.
I also had a bad condenser that created a spike in the points. And then a shot vacuum unit. Both the condenser and the vacuum unit may well have been 50-73 years old.
Cheers!
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Hi,
This site dissects failing condensors and shows where they have failed: https://nonlintec.com/sprite/cap_failure/.
Interesting to see how (cheaply) they are constructed, so no suprise they fail.
I wonder how the Distributor Doctor condensors improve upon this? Anyone care to open one up?
Regards,
Clive.
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An interesting article. Thanks for posting it.
Just last Friday I drove the MK IX to the gas station to fill the tank before the weekend events. Afterwards I got into the car to drive away and it wouldn’t start. After some fruitless effort I got out my diagnostic kit and determined that the side of the coil that was connected to the distributor was never opening. Just then a friend who lives nearby happened to see me and drove in, hooked my car to his truck with a tow strap and dragged us to his house. The condensor was shiny and had a red wire, but it was shorted. Luckily I carry a spare. I learned a couple of things, one is that even after only driving two miles the intake manifold gets hot enough to burn my arm when trying to reach the distributor. The other is that the distributor is hidden way down there, and it is a struggle to replace the little screw that holds the condensor on without dropping it into the distributor body or on the ground. By the time I had determined the problem the engine was well and truly flooded, so even with spark it took a while for me to get it started. Now I am seriously considering mounting a better condensor somewhere more convenient.
The above referenced article shows Yet Another Option, and I looked at the specs for the capacitor he uses and it is very impressive. Considering the cost of the parts and the labor involved it’s obvious that the guy who builds them is not in it for the money. I just like it when I see someone recognize a need and then fill it.
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It’s about time, Mike…
You can mount it on the rear bumper if you want! I gave up many years ago, putting them in the distributor, but mounted them either up on top of the manifold, or on the inner fender panel with a wire leading to it.
Were I motivated to keep any of my old cars with points/ condensor – though I don’t have any, if I did, I never would – I would definitely use this thing, mentioned in the article.