ignition leads

[jb]

Do I need new ignition leads?
How do I test?

I have done a bit over 60,000 miles on the HT leads that were fitted new when I got the car.

When hot the idle sometimes drops onto 5 cylinders or maybe 4 cylinders with No 1 and maybe no 2 dropping out occasionally.

There is also an intermittent 'stumble' so short in duration that I thought I was imagining it.

The plugs are relatively new and I have been running a CDI+ box since the beginning of the year.

I thought I ought to be able to test the leads and tried getting a reading on my multimeter set to ohms on No 1 lead and No 3 lead.

No 1 (suspect cylinder) shows some deflection but No 3 (ok cylinder) shows hardly any.
I am out of my depth but thought it ought to be the other way round but I might try my other multimeter which might be calibrated differently,

[Ian Comerford]

I'm no expert, one may be along later, but I understand that if your leads are breaking down they can show signs of this by something called tracking. If you start your car in the dark and look in the engine bay, tracking is the electricity running along the outside of your leads and will be very obvious (apparently).

Ian

[911hillclimber]

Check for corrosion in the plug cap.
iirc #1 is exposed to water through the grill?
I had the same issue once with my 3.2 in my '73T. Found white corrosion in the plug cap.
Bought new caps and all is well for 15 years now...

[C3s]

I remember lifting my MGB bonnet in the dark years ago, it was like a physics experiment but weirdly the car ran well??

[Jonny Hart]

Leads do degrade. C3’s physics experiment is worth performing. Start her up in the dark and have a good look around for stray sparks. I had a issue around the area of #1 spark plug and could see the odd blue flash in the dark.

Failing that, you have the internal resistors in the long spark plug caps. These tend to get carbon covered over time. You can actually repair/clean them by removing the end and taking them apart. Need to epoxy them back together though.

Also, the plug cap is screwed into the end of the lead. The cap has a spiky brass screw which pierces the lead in the centre. New leads often have the screw part not centred in the lead core.

[Ashley James]

Measure the resistance, its goes high over time. I'm guessing they start at about 10,000 Ohms.

[Jimmy]

As per the above post, measure the resistance of the leads. Disconnect either end of the lead, using a multimeter (select ohms), attach the test leads one at each end the associated lead. With the multimeter still connected, work the lead and note the reading, repeat this for all of the ignition leads. There is every chance one will be of different resistance or breaks down whilst moving the lead during the test.

You suggest you are 'green' in this area hence the detail provided. This is a simple however effective test.


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Jimmy

[Jonny Hart]

Lead resistance should be 3kOhms or thereabouts.

[jb]

Thanks everyone
Once I had put a new battery in the multimeter I got about 3k on the leads of both good and bad cylinders so perhaps they are ok after all and there was no corrosion in the plug caps.
I tested them when cold so perhaps they are different when hot.