why do cb points spark?
 

I've been doing a quite a bit working on the elecrics of the bike over the past week. Although I've learnt quite a bit from the Haynes (UK) motorcycle electrics book, I have a question.

Current goes through the primary winding of the coil, and then this circuit is broken when the points open, causing the secondary winding in the coil to generate voltage which then produces a spark at the spark plug. But why does there need to be a spark at the points when they open?

cheers,
Doug

Tis the nature of the beast... http://www.horizonsunlimited.com/ubb/wink.gif

There is a current across the points being broken - imperfectly and unevenly by the points, which are never spotless and perfect, so you will always get some arcing. Don't worry about it.

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Grant Johnson

Seek, and ye shall find.
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One world, Two wheels.
www.HorizonsUnlimited.com

If you are working on a cb honda with points beware of too much sparking. I still occasionaly have a points ignitioned machine come thru the shop. I always start them and observe the points. Some sparking will happen at low engine speeds but sparking should fade with speed increase. For service out on the journey clean and smooth the points as much as posible and then burnish them with a clean white card like a buisness card. Clean them till you get no residue on your white card.I also do this when installing new points. A bad condensor will make honda points sometimes look like a arc light.

So the spark is just a by-product of the way they work?

To be honest I never understood how coil/points worked till now. And couldn't understand what the spark was all about, as I couldn't see a reason for it. And now I know there's none.

thanks for the info....

Doug (leaving the UK tomorrow for a 3 year trip!!!)

to answer your last question, yes...

So how come we haven't heard about your trip before? http://www.horizonsunlimited.com/ubb/smile.gif

Tell me more for the ezine, and do you have a website, or do you want a blog? (See travellers stories on the left menu)

Have fun whatever, and keep us posted!


------------------
Grant Johnson

Seek, and ye shall find.
------------------------

One world, Two wheels.
www.HorizonsUnlimited.com

left London on 1st Dec, down to the south of France to wait for my driving license and carnet (which will be sent on from home). I am sooooo disorganised!! But I figured it was better to wait for documents in nice weather than in London!

Travelling down the West Africa and hopefully on to South America, then probably the 'usual' route back to the UK (Alaska/Russia/etc).

The bike is an Enfield Sixty-5. Only just bought after my Africa Twin was stolen a few weeks ago. The Enfield is now fully run in...

I'm not used to having no-where to be!

http://www.horizonsunlimited.com/ubb/smile.gif

(www.ipercolate.net)

Dougie,
the spark at the point gap is caused by the 'back-emf' of the ignition coil.the system works like this:
1) points close , current flows through points energising the ignition coil via it's primary winding.
2) voltage builds across ignition coil, which creats a magnetic field.
3) points open ,current interrupted from coil primary, magnetic field starts to collapse.
4) magnetic field collapses quickly, inducing a high voltage in the secondary winding ( creating the spark which fires your engine ).

now , since the primary winding is still present ( although not directly in circuit since the points are open ) there is a large voltage induced into this too by the collapsing magnetic field. Left to it's own devices this would be sufficient to jump the points gap as they open and burn the contacts up.
To prevent this a condensor or capacitor is placed in circuit across the points. This is of a value chosen to limit the rate of rise of the voltage across the points (by soaking up the energy) so that it can't get high enough to damage the points. Too small a value and the points will still burn (can't soak up enough) , too high and the efficiency of the ignition coil is impaired.

The 'kettering' ignition system as it is called is actually one of the earliest and simplest forms of a DC switching power supply, the principles of which are running your PC as you read this.... :)

RayT

why points spark
 

is because the capacitor is not big enough....
you might want to change to teh cdi system that still uses the points, they just pass milliamps, and in event of an electronics failure, can be bypassed in 2 mins just by changing over 3 spade connectors. This system more than trebles points life, and gives better spark. I dont normally recommend fitting anything unfixable, but it is so easy to revert to the original setup I make an exception in this case. they can be had from :=

http://www.f2motorcycles.ltd.uk/

and i thoroughly recommend these devices