[cdaly]

1991 900 N/A with EZK ignition. I'm getting some misfiring when cold, improving when warm but not going away entirely. I took off the plug cover and started it up in the dark to look for arcing. Didn't see any but, on reving, the entire lead would flash! I take it this is not normal?... There's a new distributor cap fitted and the plugs were checked and looked fine a few weeks back. I tried a set of leads from another car but with much the same flashing lead effect...

 

[ShadowWorks]

Check the earth strap from the alternator/battery to the engine, clean and maybe recrimp.

 

[cdaly]

Update. This morning (cold, lots of dew on the cars) it ran very poorly, missing on one or two at times. Once it warmed up it was better. Yesterday evening, driving home was a breeze, smooth and powerful as expected. Going to try some WD40 on the leads to see does that make a difference...

Didn't check the grounds yet though there are no other electrical issues, no slow starter or random lights or anything.

 

[cdaly]

So I changed the plugs and all seems fine this morning. No missing. The plugs I took out were NGK BCP7EV while the manual recommends BCP5ES which is what I put in. With the old plugs the car was missing a lot when cold but seemed to be fine when warmed up. Question is, is that due to the plugs being worn out (they looked fine) or due to them being too cold for the N/A engine?

 

[Matthew]

7EVs are definitely too cold for an NA. Because the plugs never get hot enough for deposits to clear, they'll missfire.

Which brand of HT leads are you using?

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[philb]

So I changed the plugs and all seems fine this morning. No missing. The plugs I took out were NGK BCP7EV while the manual recommends BCP5ES which is what I put in. With the old plugs the car was missing a lot when cold but seemed to be fine when warmed up. Question is, is that due to the plugs being worn out (they looked fine) or due to them being too cold for the N/A engine?

Aha! So I was right!

 

[SteveTheFolkie]

You still shouldn't have a visible discharge between the wires - sounds like she might be due for a new set of Bougicords ....

 

[cdaly]

The discharge seems to be within a wire rather than between them. As to what make, the originals were Bougis while the current ones are whatever was in the 'vert when I borrowed them...

As for whether the flash is still to be seen, I'll have to find a suitably dark night before I can test that again.

 

[Jim Mesthene]

You still shouldn't have a visible discharge between the wires - sounds like she might be due for a new set of Bougicords ....

...even if yours are fairly new.
If the spark can't jump the plug gap, it finds someplace else to go; often through the wire's insulation. After the first spark jumps through the insulation, it leaves behind a thin carbon track where it burnt the wire's insulation. This carbon track is the ready-made path for the next spark, which widens the path. Now your wires are compromised and should be replaced.
All the parts of the secondary ignition system are similarly compromised. That's why, when I see a spark jumping where it shouldn't, I replace the cap, rotor, wires, and plugs. Leave one of those parts on, and you have a weak link.