Author Topic: What blows up a coil?  (Read 1282 times)

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Offline jacquesleclochard

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What blows up a coil?
« on: July 25, 2013, 04:00:56 PM »
I have had this happen two times now, on a new build where i left the ignition switch on for a while. Coil heats up, and goes pop.

Stock CBs dont have this. Hence me forgetting to switch off. But how is this possible?

Offline LesterPiglet

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Re: What blows up a coil?
« Reply #1 on: July 25, 2013, 04:03:36 PM »
I've heard warnings about this on older cars. Seems it does happen.
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Offline lucky

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Re: What blows up a coil?
« Reply #2 on: July 25, 2013, 04:33:50 PM »
I have had this happen two times now, on a new build where i left the ignition switch on for a while. Coil heats up, and goes pop.

Stock CBs dont have this. Hence me forgetting to switch off. But how is this possible?


What kind of coil are you talking about.

Offline 754

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Re: What blows up a coil?
« Reply #3 on: July 25, 2013, 08:33:42 PM »
I have melted coils due to wiring regulator improperly.. Thanks to to the K2. Clymer diagram having an error..
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Offline TwoTired

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Re: What blows up a coil?
« Reply #4 on: July 25, 2013, 11:05:15 PM »
The stock coils have about 5 ohms for the primaries and they are "on" via point closure about 50% of the time, averaging about 33 watts consumption/dissipation by two coils or 17 watts each. With the engine not running, one of the points will be open and the other closed, so one coil will have to dissipate the full 33 watts and the other nada.

However, I would be more worried about burning the points contacts than overheating a coil.  However, if the coil was also in direct sun on a hot day with no airflow over the coil, I suppose it could be damaged.

3 ohm coils would be a different matter (hotter @ 54 watts each coil) and if there was a Dyna-S installed, both coils would be on and fully consuming when the engine wasn't running but the key switch was on.  Put a box around the coils and insert a cake mix for a tasty treat!

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Offline bryanj

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Re: What blows up a coil?
« Reply #5 on: July 25, 2013, 11:16:49 PM »
On a single outlet coil the ground for the HT  must go throught the casing (i think) and i do know that if the primary is wired in reverse the coil will fail--I had this happen on an Enfield Continental back in the early 70's what happened was the spark stopped when the coil warmed up.
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Offline Spanner 1

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Re: What blows up a coil?
« Reply #6 on: July 25, 2013, 11:42:25 PM »
Impossible on stock SOHC 5 ohm coils... with the ign. left 'on', one of the coils will be in circuit due to it's point being closed. The coil draws about 2.5 A from a good battery and will just deplete the battery without 'frying' itself...... now 1 or 2 ohm ( or fractional ohm ) coils as used in electronic ignitions coupled with points = smoke escape...
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Offline Vincent P

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Re: What blows up a coil?
« Reply #7 on: July 26, 2013, 07:53:10 AM »
I was always told to keep the kill switch engaged if the ignition is in the on position and the bike not running; this will save both the points and coil.
Just out of curiosity, how long are you leaving it in the on position?
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Offline jacquesleclochard

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Re: What blows up a coil?
« Reply #8 on: July 28, 2013, 02:46:45 PM »
How long?... I think 10 minutes or something? One of the coils heats up and goes pop. As in, the inside bursts trough the insulation. Pretty.

Lucky, I'm not talking about stock ones. I was using replacement coils, with the same output. Now I'm using the coils of an F2, because I am using a longer coil wire.


Offline Vincent P

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Re: What blows up a coil?
« Reply #9 on: July 28, 2013, 07:23:39 PM »
Ten minutes is a long time to have the key on you could very well be energizing the coil during this period.

I'd be curious to see if the coils react the same with the key on and the kill switch engaged???
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Offline trueblue

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Re: What blows up a coil?
« Reply #10 on: July 29, 2013, 03:56:55 AM »
If you leave the key on for 10 minutes solid with the engine not running I would fully expect there to be potential for damage from overheating.  If you flick the kill swith to the off position this cuts power to the coils and removes the issue ;D
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Offline SBR-CB400F

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Re: What blows up a coil?
« Reply #11 on: July 29, 2013, 06:27:42 AM »
Hi guys, finally decided to fit my dyna 5 ohm coils (DC8-1) to my 400f drag bike, before the drags next month. Already had the dyna ignition in. Following the install instructions, after soldering primary wires to the supplied ring terminals and screwing them to either terminal on coils, modified the mounting brackets and connected it all up replaced gas tank, flicked the ignition on ready to kick it over, thought I saw a little smoke and smelt something getting hot, the 2 black and white wires that plug together felt warm, thinking it was the primary wires that needed switching around?? retried it to realize the light in my ignition switch didn't work could this have shorted out? what have I done wrong? (Drags are on in 2 weeks)

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Offline jacquesleclochard

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Re: What blows up a coil?
« Reply #12 on: August 17, 2013, 04:44:50 AM »
Lucky, I'm not talking about stock ones. I was using replacement coils, with the same output. Now I'm using the coils of an F2, because I am using a longer coil wire.


F2 coils... An F2 doesnt use points ignition, but an electronic one. Using those coils with a traditional points ignition from a K2, that could be the cause of some malevolence, si?

Switching to an electronic ignition would cure me of this, Im inclined to think. But I need some more information. Am I off?

Offline 70CB750

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Re: What blows up a coil?
« Reply #13 on: August 17, 2013, 06:44:17 AM »
My 78 F3 has points, just saying :)
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Offline TwoTired

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Re: What blows up a coil?
« Reply #14 on: August 17, 2013, 10:51:46 AM »
F2 coils... An F2 doesnt use points ignition, but an electronic one. Using those coils with a traditional points ignition from a K2, that could be the cause of some malevolence, si?

Switching to an electronic ignition would cure me of this, Im inclined to think. But I need some more information. Am I off?

Both the Cb550F2 and the CbB750F2 used point ignition from the factory (as well as any K2 in the SOHC4 category).  The points system puts power into the coil for 53% of the time while running.
And electronic system such as the Dyna-s, puts power into the coil for 92% of the time during running.
The longer power duration can cause the coil to operate hotter, and heat is generally the enemy of electrical devices, particularly the insulation coating used for wires.  When the insulation fails, the wraps of wire short out, reducing function in the coil.

I know that some electronic ignitions operate differently than the Dyna-s.  So, whatever you select for points substitution can operate better or worse, depending on the electrical design of the unit. Specifics matter.

If the dyna-s does overheat and damage your stock coils, Dyna can also sell you their own coils to replace them.

The Dyna-s powers both coils constantly with the ignition switch on and the engine not running.  However, this should not cause instantaneous destruction of the coils, provided they are supplied by a limit of 12-13 volts from the battery.  Do expect both coils to become warm in these conditions.  I'd kind of expect the coils to have a shorter useful life, though.  But, there are too many variables to make a quantitative estimation.

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