Coil?

[Martin6]

Hi,
Please, could someone direct me to, or run me through the check for CB750K coils? I think I've read somewhere of some resistance specs that can show a damaged coil?

I guess I could swap them over, but I'd prefer not to disrupt the connections on 1&4.

My 750K6 has suddenly started firing on 1 & 4 only. I have checked the points and no current when 2 & 3 points opens. I've traced the wiring and connectors which look OK, even nipped 1/2" off the end of the HT leads, just in case.

Was running fine at the weekend. The only thing I can think is I'd noticed my horn had stopped working. I looked at it today and traced it to the handlebar switch, cleaned everything and resolved it. I'm wondering if my on/off testing for the horn might have fried the coil, because I forgot to turn off the kill switch!

Thanks, Martin

[K2-K6]

Worthwhile checking supply to tbe coil first as may have been disturbed during fault finding.

The two wires to points are the earth route from coils that are switched to make it spark.

In addition, you'll have each coil fed with positive coming into the coil. Check that first to see if its not available on 2/3 coil as that would make that set completely dead right down through the points.

Unusual to damage the coils easily, so wouldn't be initially concerned about that possibility.

[Bryanj]

Black and white leads at coil(s) should have 12v to earth with ignition on and kill switch on.
Disconnect blue and yellow wires and should be 12v between COIL contact yellow and blue to earth.
If so problem is on wires to points or on points themselves

[Martin6]

I didn't have the tank off for the horn issue, so didn't disturb the coil feeds. Subsequently, I did remove the tank to check the coil's leads and connectors visually and all looks OK. I'll do the voltage checks tomorrow.

What I did notice is both rubber grommets on the HT leads at the coil look cracked and perished. I did wash the bike after my last ride, so water may have got into the coil?

Thanks.

[K2-K6]

Quite possible from water ingress, but likely if it's anything near or around HT to be functioning with it tracking out somewhere to find earth other than the correct route via plug. That would be quite common an occurrence and so valid to check there too. Hairdryer and then silicone grease to prevent further ingress may give results.

If you started it in the dark then you'd see that flashing across to something nearby, probably here a "crack" as it did so too, but a little close to engine noise to discern.

Initially though, without a supply into the coil being verified, then its difficult to judge conclusively as to where the fault may originate.

[Pauarc]

https://youtu.be/W4HRPsTPl0A?si=gnP8oVxs_RqP18eG
Hi Martin this may help
Paul

[Martin6]

https://youtu.be/W4HRPsTPl0A?si=gnP8oVxs_RqP18eG
Hi Martin this may help
Paul

👍

[Martin6]

All 12v feeds to the coil are reading fine. Bullets are clean and tight. The resistance checked out fine on primary and secondary, then started the bike and it ran on all 4. Took it for a ride and it cut out on 2 and 3 again before I got half a mile.

I think it is the coil, I should have retested the resistances while still hot, but I'd already ordered a new coil and 2 new plug caps while I was at it, from David Silver. I'm hoping to ride tomorrow with a mate and I didn't want delivery to hold me up.

New coil arrived just before lunchtime today, 24hrs on std delivery. Was impressed. I was expecting to shove it on in 30 minutes to an hour. Took me over 3 hours. It is a pattern part and the coil mount holes are further apart than the bike frame's bracket which then interferes with the front spacer that sits on the mounting bolt. Also, the 12v feed terminals are spade connectors on the new coil, they are longer than the original arrangement, so interfere with the bike's frame. Also, the 12v connectors are both male, rather then one being female.

I made up a temporary extra bracket extension, out of a large diameter disc washer, drilled with a second hole and filed down to be bolted onto the frame bracket and then the new coil onto the extension. I don't generally have metal offcuts lying around, so I had to make do. It's solid though and doesn't interfere with the fuel tank. I had a sheathed double female connector, so that sorted the male to male connection.

But a simple job had become frustrating. I rang David Silver to check they'd sent the correct part. They had. They knew about the mounting holes being incorrect, but had never had anyone raise the incorrect bullet connection. Their view was it's a pattern part, not OEM. I could send it back for a refund if I wished. I asked him to add a note to the item on their website, to warn customers.

Anyway, bike started up from cold on all 4 nicely. Needed to feed and walk my dog, so only had time for a 20 minute run, which was fine.

I guess, no earth shattering learning here. 1. Old coils can seem OK but not cope when they warm up, 2. Pattern parts may have used a different pattern.

Sorry, bit lengthy! 

[McCabe-Thiele (Ted)]

I'm sure you have, worth mentioning though, the coils should be marked for polarity and connected appropriately.

The coils will still work but not strictly as they should, be interesting to know if the coils are branded?

[K2-K6]

Hopefully you'll get a good and uninterrupted run out on today.

Pinpointing the real cause always adds to personal knowledge store, plus research as you've done here just fills in gaps with a little more confidence in your looking after it.

To be fair, these original coil are fantastic in their reliability, possibly better than most in construction.  They do however eventually succumb to flexing around the HT lead to coil area, ultimately to crack or disconnect the HT route continuity.

There's, on here, Ash's coilfix method to replace the HT and reinstate them.

It's possible that if the HT are trimmed to fit new caps over the years, then the eventually get too tight when pulled to get them onto plugs that may play a part in this effect.

[Martin6]

Went well. 100 lovely miles. 👍

[Johnny4428]

Excellent Martin you’ve sorted the problem and ready for some descent runs over the season. Very annoying that when you buy a replacement part that you have to fettle to make fit.😡

[McCabe-Thiele (Ted)]

I would have expected a DS replacement part to be a straight fit, that why it's often worth paying his prices.
I thought at one time he sold the original brand name coils for some models.
I expected to have to make some alterations when I fitted by Dyna coils to my 400. Likewise with the cheap Hendler coils to my 500.

[taysidedragon]

I bought 400f coils from DSS a good few years ago and they were a straight fit onto the original mountings.
He's probably not able to source the exact coils to fit at times so has to buy the next best alternative.
Should make it clear that the fittings need modifying though.

[Martin6]

I've used them a fair bit on my rolling restoration, especially when it wasn't rolling! 1st time I've had anything but good to say about them, perhaps apart from pricing on some difficult to find elsewhere parts. On the whole, I'm a happy customer.

Sun is out, 20 degrees C, will be out again this afternoon