CB750 coils on a XS400?

[Stabler]

So I need to change out my coils on my XS400 because I left the ignition on over night and probably fried one (or both) my coils. Stupid-stupid..

The question is, could I use two coils from my stash of CB750 parts and run just one spark plug out of each? Anyone know? The CB750 coils are 5 ohm as well and I have four of them just laying around.

Yeah I know "get a set of new ones - yada yada yada", but I'm cheap and I'm going to place them under one of the sidecovers anyways so no one will ever see them (except me from time to time).

Wadda you guys think?

 

[bcware]

No. Leaving the ignition on over night will just kill your battery. Why would you think your coils are fried?

I believe the coils from a bigger bike, often, will not work. They need to be the right number of ohms; it's been posted on here before. Maybe it was 3 ohms? I forget!

 

[Stabler]

Well, the "wax" inside one of mycoils have melted and come out of the coil. That's why I thought it was fried.
I thought the XS400 had 5 ohm coils?

 

[bcware]

You might be right; I just can't remember.

Unless the bike was running all night leaving the ignition on didn't melt them; heat from the engine could though. The coils don't do anything unless the motor is turning over.

Mikesxs sells coils for the xs360/400 listed as 4 ohms. It would be best to get confirmation.

 

[bcware]

You can always re-seal the cable; I'd think it would be normal when one swaps out new ones.

Will the bike not start? What is the battery reading?

 

[PT_247]

I have heard that just leaving the ignition on with the engine not running will melt the coils, can't remember who it was who said but it was from on here!

 

[bcware]

I've never heard of that before. So you're suggesting that the coils will melt before the battery dies? That seems a bit fast to me. If it were true we would hear about it all the time. Maybe it's possible.

 

[PT_247]

I tend not to leave my ignition on unless am riding so have not even felt my coils to see if they get warm. I could be mistaken but am pretty sure I read it on here!

 

[PT_247]

yup, found the thread:

that is caused by leaving the ignition on and not running the bike, they get really hot and break doing that. Its the wax protecting the primary and secondary coils!

Oh crap. And yes, I did forget to turn the ignition off over night once this week. Well, I'm getting a new set of coils anyways. Just didn't wanna have to do this until after this weekend..

Thanks drewpy.

i only know that coz I did it

ty for the tip drewpy, will avoid leaving my ignition on.

 

[Stabler]

That thread is why I'm posting this thread. So my question remains. Does anyone know for sure if I can or can't use my Cb750 5 ohm coils? Drewpy maybe? Or xschris?

 

[bcware]

Hm, I didn't think coils worked that way. I thought it was the induction and subsequent collapse of the magnetic field that caused electrons to move and hence generate a current and therefore heat as a byproduct. Without the motor turning over the field doesn't collapse and there is no current, just a magnet.

It's hard to find a good explanation of just this process. I certainly can't find a guide that explains what's happening to the electrons at any given moment

I need a physicist or engineer

 

[Stabler]

And I need to know about these freggin' coils...

 

[bcware]

What's the bike doing anyway? You never actually said what the problem is other than melted wax.

 

[drewpy]

the coils are a big, well, coil of wire. they are held in wax and like a 1 bar fire will heat up, melt the wax and eventually blow into the secondary coils. Yes the battery will go too.

Even if the coils work, they won't be reliable. that's why I had to renew mine.

try the cb coils if you want, they either work or don't.

 

[Stabler]

I dunno. The tank is getting painted so I can't try it and I haven't fired it up after I saw the melted wax. The thing is I was thinking of switching them out anyways to have them hidden behind one of the plastic covers. But time is running out and I have a booked inspection by the swedish equivalent of the DMV, so I thought I'd fix this in the while waiting on the tank and fender.

I really hope someone knows something so I can get going with this.

 

[xschris]

The 77-79 points set up used a 4ohm coil the 80-82 used a 3ohm coil using a 5ohm on a tci ignition might do some harm. I would try what you have or find some on ebay or on the forum they don't cost a lot for good used sets.

 

[Stabler]

Ah - then I know. Big thanks guys!!

 

[XSLeo]

The CB750 coils have two plug wires in them. to make them work you will need two. Each one you hook one plug wire to a plug the other to ground.
That is if the ohms match your stock coils. Points bikes use coils with a 4 to 5 ohms primary. Yamaha's TCI use a 2.5 ohm coil. Using the TCI coil on the points will burn points quicker, more current draw. The points coils used on the TCI will draw less current and probaly produce a weak spark.
On a TCI bike the TCI has a shut down circuit that cuts power to prevent damage to the TCI and coil. Point don't. A point bike if left on and the points are closed, power will keep flowing through the coil primary side. This creates heat. When running the coil being turned on and off keeps the heat under control. With it not running the heat isn't controlled and the coil will over heat, this can and will destroy a coil. I did this and the heat cracked the case and melted the insulation on the wire in the coil, shorting it out.
Leo

 

[bcware]

That explains everything; thank you!

 

[Stabler]

Thanks for all the answers guys! New coils it is then.. Or a new coil anyways. Anyone got one they're willing to part with?