# Motor bike coil terminals



## deverett (Jan 5, 2015)

I've got a 12v motor bike coil ex Honda CB125 (I'm told) that I would like to use in a model slow revving 4 stroke engine.





Questions.  
1. How do I know which of the two small wires is the + and which is the -  ?  There are no markings on the case.  Presumably, when used for its intended purpose, the male and female bullet connectors would remove any confusion.
2. It being taken from a scrapped bike, is there any simple way with a test meter to tell if it is OK?

Thanks

Dave
The Emerald Isle


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## Jasonb (Jan 5, 2015)

The live wire on the loom would likely have had an insulated bayonet fitting so that would go to the uninsulated wire on the coil.

Are you making a buzz coil from it? or just wanting a small HT coil

J


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## dlhoulton (Jan 5, 2015)

The Black wire with the white stripe is the positive (hot) wire. The yellow wire would go to the points condenser. On the wiring harness where the coil is mounted there would be black/white wire feeding power to coil and a yellow wire for your points and condenser.
With a simple multi meter you could set it to read Ohm's (200) and connect one probe to one wire and the other probe to the other wire and depending if the coil is a 3 or 5 Ohm coil get the reading. I have two Honda CB750's and that's how I do a quick test on them.
Here is a quick youtube tutorial on how to test/check a coil.
[ame]https://www.youtube.com/watch?v=rRQ45lT2d3c[/ame] 
Hope this helps.


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## canadianhorsepower (Jan 5, 2015)

dlhoulton said:


> The Black wire with the white stripe is the positive (hot) wire. .



 this might sound strange :wall: but remember this is the picture of the coil disconected from the harness so the black wire can't short on the bike it's surounded with the sleeve Thm:


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## deverett (Jan 5, 2015)

Thanks for the replies and the video link.  I can understand the rationale that the open bullet is the live wire.



Jasonb said:


> The live wire on the loom would likely have had an insulated bayonet fitting so that would go to the uninsulated wire on the coil.
> 
> Are you making a buzz coil from it? or just wanting a small HT coil
> 
> J


Actually it is a replacement for a burnt out (I think) coil.  The circuit being used is this one:




For me with no electrical knowledge, it's easier to try a replacement coil than to test the transistor (although I do have a spare transistor if that is needed).

Dave
The Emerald Isle


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## dlhoulton (Jan 5, 2015)

Just incase you wanted to know that coil is original (OEM). Those "bullet" connectors are standard on all CB's in that era. The male bullet had a small insulator/silicone piece at its base so that when plugged into the female (which had a larger insulator) it would provide protection from shorts, moisture, etc. Also it provided a "plug n play" type way of making sure they didn't get crossed. Here is a pic of a CB750 where in the wiring harness they would have plugged into (CB125 would have only one black/white and only yellow). Also here is a pic of a set of coils (again CB750) to show that the plug (bullets/insulators) were same in all CB's of that era.


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## deverett (Jan 5, 2015)

That's interesting.  Among the bundle of motor bike coils I got is a double mounted one, like the one in your first pic.  I separated them, 'cause I only anticipate running single cylinder engines.

Dave
The Emerald Isle


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## MachineTom (Jan 6, 2015)

I will take a stab here, I was a Honda motorcycle dealer for many years, a CB 125 was likely a low voltage magneto driven ignition and did not have points. Pending a schematic to the contrary anyway. Those exposed bullet ends of the coil went inside a female sleeve, that was insulated. So don't think one goes to ground as they don't. 
Use a ohmeter and see which wire has the lowest resistance between the spark plug wire, and the low voltage wires, that would be ground through the points. the other to your battery.

An ignition coil will not make a spark when connected to a battery, It makes a spark at the instant it is disconnected from the power. Which is what points do, they break the circuit, the magnetic field collapse's and induces a current into the secondary coils, and that is the spark at the plug that you see.


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## Theclockworks (Jan 6, 2015)

Have to disagree with you Tom all CB 100/125/TL 125 where and still are including Chinese modles conventional points and battery system powered by a alternator/rectifier . (X Honda GB employee for 30years)also the spark is generated through the condenser  to the points which I turn control the spark through the HT circuit the points are open and closed by a lobe on the end of the cam shaft giving you a dewell for ing timing the points gap is normally 15 tho.


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## deverett (Jan 6, 2015)

Thanks for all the helpful replies.

Following the instructions in the video, I put the tester across the 3 leads in turn.  Nil readings between the two LT leads and also nil between the HT and one of the LT, so it seems that all 4 coils that I had been given were duds.  Maybe that's why I was given them!

Now another question.  I need a working coil and the cheapest that I can find on eBay is described as 12v ELECTRONIC IGNITION COIL, Item no. 350799327676 or do I need a traditional one like 301274822292.  Is there any difference as far as running with a low RPM single cylinder 4 stroke engine.  The ignition system is rigged up as per the circuit diagram above.

Dave
The Emerald Isle


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## Theclockworks (Jan 6, 2015)

deverett said:


> Thanks for all the helpful replies.
> 
> Following the instructions in the video, I put the tester across the 3 leads in turn.  Nil readings between the two LT leads and also nil between the HT and one of the LT, so it seems that all 4 coils that I had been given were duds.  Maybe that's why I was given them!
> 
> ...



The only difference being E I C coils need a good consent 12v supply, you may find the quality of the spark may drop off and some times  damage spark plug if volts drop down below 12.5/12Vs


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## BaronJ (Jan 6, 2015)

Hi Dave,  I suspect that cutting the pair of coils apart may have damaged them.  It might be easier to just grab a coil from an old car at the scrap yard, plus if it looks OK then it probably is.  As a bonus it will likely have three terminals that are easily identified. Battery, Plugs and points or what ever the car it came from uses instead of points.  Google &quot;Kettering ignition&quot;  wikipedia.org/wiki/Ignition_system,  has a nice diagram.


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## deverett (Jan 6, 2015)

I'll look out for an old coil, then.  If I can't find one soon, looks like I'll have to dig a bit deeper into the pocket.

Dave
The Emerald Isle


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## BaronJ (Jan 7, 2015)

deverett said:


> I'll look out for an old coil, then.  If I can't find one soon, looks like I'll have to dig a bit deeper into the pocket.
> 
> Dave
> The Emerald Isle



Pity you are not nearer !  The local scrap yard has a dozen cars in at the moment and are currently breaking them.  Most if not all are around ten or so years old, so will have less electronics than a modern car.


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