Distributor cap shorting out?

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Napoleon Boot
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Distributor cap shorting out?

Post by Napoleon Boot »

Hi everyone,

I've lurked here a long time and posted in the other place, but this is my first post here, so hello!

A few weeks ago my Moggie (1971 Traveller, 1098 engine) started missing a beat occasionally when I put my foot down. I didn't worry about it too much at the time.
Meanwhile, my exhaust pipe had developed a crack (old age), so I replaced it, fitting a new gasket and clamp at the same time. The car got a bit of power back, but I noticed that the misfiring was now happening a bit more often. The car had always run a bit rich, and I thought that perhaps, having always had a leaky manifold and knackered exhaust, the newly fixed engine needed tuning up. I leaned out the fuel mixture and checked it with the lifting pin, all seemed OK.

Then the trouble started. The misfiring got worse and the engine sounded rough at idle. It would run OK, but misfired whenever I put my foot down more than a touch, and whenever we encountered an uphill gradient, the misfire would get so bad that the car lost all power. At one point I had to be rescued by kind passers-by who gave me a shove and got me up the little hill into my street.

I worked my way round all the items that might cause misfiring. Swapped spark plugs, checked plug gaps, checked HT leads, checked electronic ignition module, replaced the tatty fuel line, checked the fuel pump, checked the carb piston, needle, topped up dashpot oil etc. I reset the carb and tried tuning from scratch, checked with a Colortune - flame colour looked OK.

Then today, as I took the HT leads out of the (25D) distributor cap, I noticed that a hole has appeared, bored from the end of the lead to number one cylinder through to the inside of the cap. I've attached a photo (not easy photographing a black hole inside a black plastic cap).
Distributor cap.jpg
Distributor cap.jpg (409.13 KiB) Viewed 40 times
Distributor cap.jpg
EXIF-Data show/hide
Original image recording time:
2021:07:17 14:59:11
Image taken on:
Sat Jul 17, 2021 1:59 pm
Focus length:
3.99 mm
Shutter speed:
1/219 Sec
F-number:
f/1.8
ISO speed rating:
20
Whitebalance:
Auto
Flash:
Flash fired, compulsory flash mode
Camera-manufacturer:
Apple
Camera-model:
IPhone 7
Exposure program:
Normal program
Exposure bias:
0 EV
Metering mode:
Spot
Does it seem possible that the spark is shorting out through this hole somehow, when the engine's under load? I've searched but I can't find any threads where this has happened in the past.

I hope this is the culprit, as the dizzy cap is still under warranty and it would be a nice easy fix! Would love to hear your expert opinions.

Best wishes,
Seb



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Sleeper
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Re: Distributor cap shorting out?

Post by Sleeper »

Do you have a multimeter?
If it was arcing through there a resistance reading may prove it..

John ;-)



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Napoleon Boot
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Re: Distributor cap shorting out?

Post by Napoleon Boot »

Thanks John,

Yes, I do - I'm only used to working on low voltage though - my guess is that the high voltage current is arcing from the end of the lead when the rotor arm is touching the brass stud, and it's shorting on something. I'm not sure how I'd test that with the multimeter though.

Seb



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Re: Distributor cap shorting out?

Post by Sleeper »

I'd compare the resistance measured between the hole and the electrode ( the one in question ) on the highest scale you have and compare it to the non-existent hole and other electrode.
If my theory is correct you should have infinity on the latter and a reading on the first...

ie. the path of shorting will have carbon deposits which the meter should pick up?

John ;-)



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Napoleon Boot
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Re: Distributor cap shorting out?

Post by Napoleon Boot »

Sleeper wrote:
1 week ago
I'd compare the resistance measured between the hole and the electrode ( the one in question ) on the highest scale you have and compare it to the non-existent hole and other electrode.
Ah, I get you, thanks John.

Well I can mark this one as solved - just as I was resigning myself to a couple more days without a car, I remembered that there's an old-fashioned motor factors not too far from me. I went down on my bike, picked up a cheap, pattern 25D cap, and came back and fitted it.

Worked first time! I've just been up and down a few big hills to celebrate, no misfiring and the fuel mixture seems OK too. Tomorrow I'm going to seek out some bigger roads where I can go faster than 30.
Aren't high street shops useful? We'll miss them when they're all gone and everything is mail order.

Seb



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