Life of HT leads / HT caps

Helpful information and requests for assitance and advice
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thornebt
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Life of HT leads / HT caps

Post by thornebt »

My G80 wasn't particularly easy to start the other day. It has a brand new spark plug but I was wondering if there could be a problem with the condition of the HT lead and cap. I tried testing the spark by holding the plug near to an earth and kicking the engine over. It's not particularly easy when you're leaning over the bike whilst kicking it over and trying to hold the plug in a suitable position, especially when trying to see the spark on a bright day!

The lead and cap are certainly over twenty years old. Would these copper stranded leads break down after a long time? Is it practical to test the resistance with a multi meter? Likewide with the HT cap, would that deteriorate with time?

Cheers. Bruce.
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Rob Harknett
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Re: Life of HT leads / HT caps

Post by Rob Harknett »

If the plug cap holds firm that should be OK, you could perhaps cut the lead back a bit and re screw it in. Then check where it goes in the mag. I think you have N1 mag as I see you have 47 engine. Undo the lead at the mag end, pull out the spring and carbon brush. Check the brush is not worn. Replace if OK making sure it moves freely. Make sure the wire strands are fanned out over the little washer then screw the lead back in. Now give it a try knowing you have checked all connections, the brush not worn out and moves freely.
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thornebt
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Re: Life of HT leads / HT caps

Post by thornebt »

Thanks Rob. I'll check those things. Can anyone tell me though if the lead might have deteriorated and could be worth replacing? Cheers. Bruce.
Mick D
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Re: Life of HT leads / HT caps

Post by Mick D »

Hi

For the sake of a few pounds I'd replace both the lead and the cap - I sourced a nice sympathetic looking NGK cap from a local motor factors for £3 and the lead was less than £1 per foot from AMC Classic.

The leads can deteriorate - it's usually caused by the insulation breaking down with age or conductor suffering mechanical damage where the terminations are fitted.

Regards Mick
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dave16mct
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Re: Life of HT leads / HT caps

Post by dave16mct »

Fit a non-resister cap. NGK do a nice non-resister one. The standard NGK cap has a resistance of 5K Ohms. Not what you want on a magneto bike.
Dave.
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thornebt
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Re: Life of HT leads / HT caps

Post by thornebt »

Thanks for the advice chaps. I'll change the HT lead and caps. There's no point in spoiling the ship for a 'apence worth of tar - or whatever the saying is.

Cheers. Bruce.
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clive
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Re: Life of HT leads / HT caps

Post by clive »

thornebt wrote:Thanks for the advice chaps. I'll change the HT lead and caps. There's no point in spoiling the ship for a 'apence worth of tar - or whatever the saying is.

Cheers. Bruce.
Don't get the tar in the mag, it will ruin it.
clive
if it ain't broke don't fix
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kernow kid
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Re: Life of HT leads / HT caps

Post by kernow kid »

Hi Bruce, testing the resistance of the lead is a bit of a non starter, no pun intended! You'd need a special low ohm meter and regular multimeters don't offer that resolution for checking resistance. Also, as mentioned, it's the insulation that's likely to break down when the lead is under load, so a lead swop is much easier. One weak point is where the cap screws into the lead. It's basically a self tapper screw pointy end that winds into the copper core of the lead, so make sure it's screwed fully on and a little bead of black silicon sealer where the cable enters the cap should help prevent water creeping in .
Regards kk
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Groily
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Re: Life of HT leads / HT caps

Post by Groily »

For what it's worth I reckon changing leads and caps every few years is a very good idea.
The inner core of the HT cables can get quite grotty over time at both ends of the lead, and the insides of plug caps too. And the outer sheathing does deteriorate, on some types of cable more than others.

Yes, it's hard to measure insulation leaks with a normal sort of meter, but from end to end of the 'metal' bits of a lead and cap you want to see zero ohms or as near as, with a mag. That you can check well enough with a cheap meter no problem even though it might not show '0' ohms. With a battery/coil system the suppressed caps are usually fine, with their 5K or even 10K ohm resistors in - but they too can suffer from crud and corrosion, and the self-tapping screws can self-detach inside, especially on Champion Butterfly ones.

Personally I believe in soldering the little brass split washers on at the pick-up ends of all HT cables and filing the end flat and shiny, with no up and down movement when the acorn nuts are done up (to prevent arcing, damp causing verdigris in there, etc etc.) Every little thing helps.

The other thing that does for cables is touching cyl heads etc and getting blistered or burnt. Any of that, and I'd bin them. Better a less tidy wide route to the plug(s) that keeps them out of contact with excess heat than a tidier one which scorches them. There are loads of machines out there with damaged cables . . . .

And then there's the ultimate no-no - letting the HT cables (or low tension wires on a distributor for that matter) direct any drips from a well-tickled carb into a place where sparks could lurk. That happens more often than those who haven't ever had it happen might think, quite apart from the fact that petrol probably doesn't do some of the materials any good either. Consequences can be too dire to think about. I have friends who will go nowhere without a fire extinguisher of an appropriate sort attached somewhere to their machine . . . Just saying.
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thornebt
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Re: Life of HT leads / HT caps

Post by thornebt »

I'm pleased to report that a new HT lead and cap seem to have sorted my starting problems. I put my multimeter on the old lead and cap and resistance was about 9 ohms. With a new lead and cap the resistance was about 9 kilo ohms! The bike now seems to start OK. I can't believe that I didn't consider changing these before!

I only had a resistive HT cap but a non-resistive one is in the post to me so I'll swap it later. Thanks for your input everyone. Cheers. Bruce.
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