Who needs Christmas lights when

[a8hex]

In my on going pursuit of the miss fire on my XK150 I'd noticed a few weeks back that I could see a mass of sparks in and around the distributor cap when I looked at it at night. All very pretty, but not too healthy. So I ordered up a couple of replacements and since we don't seem to be having a winter this year, and besides any salt put onto the roads would be washed away in no time, I went for a boxing day drive (boxing day seems to be like a Sunday, but more so).
There was less of a misfire but it's still there so I waited for dark and had another look.
It's very pretty, I could see a ghostly apparition of about half the HT leads, and if you touched them the glow would extend around your finger tips. There are also an interesting spark from one of the leads where it met the distributor cap down to the retaining clip, not only could I see the spark, I could clearly hear it over the noise of the engine and it's slurping of air.
But the thing that really surprised me was there was a definite purple corona around the low tension terminals of the coil.
I guess I need a new set of HT leads, but is it normal to get this effect around the coil? The HT lead I could understand, but anyone any idea about why I should see this on the low tension sides?

I tried to video it all on the phone, but the sensor wasn't picking any of it up.

[Hooli]

Northern lights?


Seriously, never head of it on the coil either, could it be the HT from the king lead earthing through the LT circuit or coil case?

[a8hex]

Hooli said:

Northern lights?

I guess it's more likely St Elmo's fire.

Hooli said:

Seriously, never head of it on the coil either, could it be the HT from the king lead earthing through the LT circuit or coil case?

I guess it must be related to the HT field from the lead to the distributor, but glow was around the tips of the LT leads. Thinking more about the physics here, I guess their shape is much more conducive to these effects. These things tend to form more around pointy shaped conductors.
The glow around my fingers was eerie, I couldn't feel anything. Once in my youth I pulled an lead from a spark plug on a running engine. This is something you only do once after picking myself back up off the floor I distinctly remember thinking that I shouldn't try that one again.

I wish I could have videoed the glow. But the phone wasn't even picking up the sparks.

[OldGermanHeaps]

Bad engine earth strap or too big a plug gap?
If the plugs present a high resistance the current will look for an easier path to earth.

[tapkaJohnD]

+1 for Oldgerman.

I had a charging problem, that was finally revealed to be a bad (=no) engine earth by the corona around the alternator.

John

[a8hex]

Well I finally got off my lazy arse and swapped the HT leads, well the ones to the spark plugs anyway. I'd bought a set for a series II XJ6 since that's the engine and distributor I've got, but the coil on mine is obviously not in the same place as the XJ6 so the lead to the coil wasn't long enough. Firstly it started far easier than in the past which was a nice change. None of the funny hiccoughs none of the distinctive click noises of a big fat spark. Switched off the lights and couldn't see anything to start with. Eventually as my eyes adjusted there was a faint glow along the lead to from the distributor cap to the coil (which is still the original one) which became more pronounced if I gave the throttle linkage a prod to rev the engine a little. I guess I need to try a get a replacement for that lead too. Oh and find a drier day to take her out for spin and see whether she's behaving on the road now.