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Spark plug cap won't screw into HT lead

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prizzle
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Joined: 24 May 2020
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PostPosted: 20:16 - 24 May 2020    Post subject: Spark plug cap won't screw into HT lead Reply with quote
Had issues starting a new bike that I bought a couple of days ago, after convincing myself it was an issue with the spark plug cap I took it apart from the plug and the lead, the end of which was dirty and corroded, I snipped about half a cm off and when resting against the cap whilst attached to the plug, the bike starts, however after that I have tried to screw it into the lead, and all it does is turn over... could this be an issue with the cap or has snipping the end of the coil just messed it up meaning I need a whole new cable?
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doggone
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PostPosted: 20:19 - 24 May 2020    Post subject: Reply with quote
Possibly. 'leaky' HT leads are a common problem with older bikes anyway and not expensive to replace.
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Riejufixing
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PostPosted: 20:25 - 24 May 2020    Post subject: Re: Spark plug cap won't screw into HT lead Reply with quote
prizzle wrote:
I snipped about half a cm off and when resting against the cap whilst attached to the plug, the bike starts, however after that I have tried to screw it into the lead, and all it does is turn over... could this be an issue with the cap or has snipping the end of the coil just messed it up meaning I need a whole new cable?

When you screw in the HT cable, does it screw in nice and solidly? Perhaps, when you hold the HT lead against the cap (the screw for the HT lead??) it pushes the screw in, but when the cable's actually attached, it pulls it out a bit and a possible gap's too much for the spark to get over. What happens if you screw it in, then push the cable in as you crank the engine, or maybe tape the plug lead direct to the plug's HT terminal for an experiment?
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prizzle
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PostPosted: 20:31 - 24 May 2020    Post subject: Reply with quote
doggone wrote:
Possibly. 'leaky' HT leads are a common problem with older bikes anyway and not expensive to replace.


Do you know if I should have left some coil sticking out the end of the cable? Ive just pushed and twisted the cap down the centre of the coil (never had to replace a cap before)
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mentalboy
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PostPosted: 20:31 - 24 May 2020    Post subject: Reply with quote
If you have snipped the cable back to clean then you should be getting better results than you started with, not worse. Are you sure that the cap is screwed onto the cable tightly?

As above, it's cheap and easy to replace cables.
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mentalboy
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PostPosted: 20:33 - 24 May 2020    Post subject: Reply with quote
prizzle wrote:
doggone wrote:
Possibly. 'leaky' HT leads are a common problem with older bikes anyway and not expensive to replace.


Do you know if I should have left some coil sticking out the end of the cable? Ive just pushed and twisted the cap down the centre of the coil (never had to replace a cap before)


Just snip flush and screw cap on.
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prizzle
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PostPosted: 20:35 - 24 May 2020    Post subject: Re: Spark plug cap won't screw into HT lead Reply with quote
Quote:
What happens if you screw it in, then push the cable in as you crank the engine, or maybe tape the plug lead direct to the plug's HT terminal for an experiment?


Pushing the cable in when everything is screwed and attached gives it connection sometimes it just depends on angle, I just find it strange that when gently resting with no secured connection I have no problems but when its a centimetre into the cable and snug I have issues.
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prizzle
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PostPosted: 20:39 - 24 May 2020    Post subject: Reply with quote
mentalboy wrote:


Just snip flush and screw cap on.


But I'm correct in saying the cap threads into the centre of the coil?
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Riejufixing
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PostPosted: 20:59 - 24 May 2020    Post subject: Reply with quote
prizzle wrote:
mentalboy wrote:


Just snip flush and screw cap on.


But I'm correct in saying the cap threads into the centre of the coil?

The coil has an HT wire coming out of it. Sometimes the HT wire is replacable. Sometimes it's part of the coil. The HT wire has a copper core (sometimes carbon "string") which connects the coil to the sparking-plug cap, which often has a screw in it, but sometimes has another sort of connector. The plug cap often has a resistor in it between the end of the HT wire from the coil and the HT contact of the plug itself, which could be between the screw connector and the top of the inside of the plug cap.

So the HT cable you've snipped a bit off isn't the actual coil, it's just connected to it. The resistor in the plug cap or the carbon "string" if that's what's inside the HT cable instead of copper wire is to suppress inteference to radio/tv signals. Sometimes these things go wrong and prevent a proper spark!
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Riejufixing
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PostPosted: 21:05 - 24 May 2020    Post subject: Reply with quote
Here you are, this is what's probably in your sparking-plug cap. Edit: The second pic has a different sort of attachment for the HT cable that comes out of the coil:
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prizzle
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PostPosted: 21:40 - 24 May 2020    Post subject: Reply with quote
Riejufixing wrote:
Here you are, this is what's probably in your sparking-plug cap. Edit: The second pic has a different sort of attachment for the HT cable that comes out of the coil:


Thankyou! I think as I screwed the cap in ive pushed some of the wires to the side and jumbled things around, I think if I cut any more off it will be too short to reach the cap so will just look into replacing the whole cable, cheers for the help guys Smile Smile
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mentalboy
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PostPosted: 23:31 - 24 May 2020    Post subject: Reply with quote
prizzle wrote:
Riejufixing wrote:
Here you are, this is what's probably in your sparking-plug cap. Edit: The second pic has a different sort of attachment for the HT cable that comes out of the coil:


Thankyou! I think as I screwed the cap in ive pushed some of the wires to the side and jumbled things around, I think if I cut any more off it will be too short to reach the cap so will just look into replacing the whole cable, cheers for the help guys Smile Smile


The screw prong on the cap (as shown nicely in Reijufixing's pics) is designed to be screwed up into the wire fibres at the centre of the HT lead. It will obviously disturb them but it is designed to do that.
The question is how clean did the wire at the centre of the HT cable look after you had clipped it back? Any green or black residue from corrosion or rot will give poor conductivity. If the bike is fairly old then I'd suggest replacing the caps as well as the HT leads - assuming the leads aren't the type that are permanently fixed to the coil or you'll be replacing them as well.
Always helps if you give us the model and year of the bike. As a rule. I have noticed that smaller engined bikes tend to have HT leads that do not remove from the coil.
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prizzle
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PostPosted: 14:51 - 28 May 2020    Post subject: Reply with quote
mentalboy wrote:

The question is how clean did the wire at the centre of the HT cable look after you had clipped it back? Any green or black residue from corrosion or rot will give poor conductivity. If the bike is fairly old then I'd suggest replacing the caps as well as the HT leads - assuming the leads aren't the type that are permanently fixed to the coil or you'll be replacing them as well.
Always helps if you give us the model and year of the bike. As a rule. I have noticed that smaller engined bikes tend to have HT leads that do not remove from the coil.


Had to leave it for a few days while I've been at work, luckily had someone come and test all fuses and connections to find out it seems to be a problem with my CDI! I have a Pulse Adrenaline 125, was just a temp bike to get me to work on until test centres are back up and running and it's caused more problems than it's even going to be worth Mr. Green cheers for the help guys... anyone reading this for help it was as simple as screwing down the centre of the cable.
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