How to test a HT lead.

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How to test a HT lead.

Postby Quello Serio » 02 Nov 2017 13:13

Can anyone tell me how I can test a HT lead?

I'm trying to troubleshoot a missing spark on my rear cylinder. Thanks to an old post on this forum, I've found that the lead screws out (not glued in like I had assumed). Is there a min/max resistance I should be looking for?

In the blue book, there was a suggestion to swap the red wires between transducers. I did that, and found that the working cylinder remained working. I'm taking this as a sign that the pickup is good. I've also got a shock off the not-working lead, and am taking this as an indication that either the transducer is sending weak current, or the HT lead is faulty.

I'm reluctant to unscrew the known good lead, for fear of breaking something.
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Re: How to test a HT lead.

Postby corsaro chris » 02 Nov 2017 18:19

If you got a shock off the non-working lead :shock: (presumably while holding it or something?) you know that there is at least energy going along it, so it's likely to be a faulty plug or plug cap... That presumes that it used to work previously and nothing has been altered to affect timing, etc.,

Good hunting,

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Re: How to test a HT lead.

Postby MickeyMoto » 02 Nov 2017 18:20

How old are the leads? My Excalibur ran on one in the wet. The 20 year old leads had decayed. I changed them for new ones and no problems since. Use good copper cables.
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Re: How to test a HT lead.

Postby Quello Serio » 02 Nov 2017 21:39

Thanks for the replies.

It did work previously. Last week in fact. Didn't start, but there was a spark on both cylinders. The leads are original, 1980, as are the transducers.

I found that at least part of my problem was lack of continuity across the crimped connection. I've got continuity back thanks to solder, but I can't re-crimp, so I'll replace them. I had been led to believe (from youtube) that there would be some resistance across the whole lead, but that doesn't seem to be the case, not on these leads anyway.
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Re: How to test a HT lead.

Postby Quello Serio » 11 Jan 2018 12:41

I followed Paul Compton's instructions (https://www.youtube.com/watch?v=CWgLYidtHVw) which showed that both my transducers were lighting the bulb, but one wasn't creating a spark (at least not one that I could see). Fitting new HT leads (NGK, 5K) made no difference.

I didn't have the skills or the facilities to troubleshoot further, so I guessed that the transducer was at fault. NLM's option was a bit pricy for me, so I ordered a replacement transducer - NRC31 (32.3921) from Nuova Ray @ €65 +€25P&P, and much to my delight this has fixed my missing spark problem.

The new transducer has no markings or labels, except for the 'HT' output being marked as 'HT'. So you're not made aware that the wiring is different from the original transducers. Luckily, wiring it incorrectly didn't do any damage. The correct wiring is as shown on the Dutch club website: http://www.motomoriniclub.nl/nuovaray1.jpg
The HT lead provided with the new transducer was also too short, but I didn't need that anyway.
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Re: How to test a HT lead.

Postby Steve Brown » 11 Jan 2018 22:29

I'd suggest you might need a new HT lead, as the description you give sounds like a suppressed lead. It's the 5k you mention, it appears on suppressed plug caps too and they are also worth avoiding. You probably guessed already it refers to the resistance in the cap or lead. In this case it means 5000 ohms. The original ignition is not really high energy by todays standards and unless you have a really good set of ignition parts right from the flywheel to the spark plugs, a resistor will have a bad effect. Most people avoid resistor plugs too but in my ignorance I got away with it for years, till aging electrical bits started to gang up on me.
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Re: How to test a HT lead.

Postby mgelder » 12 Jan 2018 12:10

On a related note, does anyone know of a source of reliable, robust and long-reach 0 ohm plug caps?

I recently replaced the two mismatched ones on my 350K with 5k ohm NGK ones and the bike runs fine, but I'd rather fit 0 ohm ones if I could find decent quality ones that reached the plugs...
Morini stuff on RealClassic.co.uk: http://www.realclassic.co.uk/profiles.html#morini
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Re: How to test a HT lead.

Postby mikeybizzle » 12 Jan 2018 13:07

mgelder wrote:On a related note, does anyone know of a source of reliable, robust and long-reach 0 ohm plug caps?

I recently replaced the two mismatched ones on my 350K with 5k ohm NGK ones and the bike runs fine, but I'd rather fit 0 ohm ones if I could find decent quality ones that reached the plugs...


Have a look at Vehicle Wiring Products they have everything to make your own .
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Re: How to test a HT lead.

Postby 3potjohn » 12 Jan 2018 20:04

VWP parts look like IT103 & RS221 to me.Guess you'd want to seal around the tops. I use resistor NGK caps now and cannot find my old plug leads which were zero resistance.Not like me to throw old bike bits away, as I am often told.
Another simple project then, once I've finished repairing the driveshaft on my old clunker.
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