K2F Magneto Cut Out Not Working

[DavidHulse]

I'm almost ready to attempt a first start after rebuilding my G11CS from bits but, before I do, I want to make sure I can stop it if it goes pear shaped or starts spewing oil. But when I connect the magneto cut out contact to earth, I still get sparks at the plugs. I've circuit checked the cut out contact to the carbon brush, the mag body, the engine, and the frame; and all appear to be correct. Does anyone know if this is normal, or is there a common fault that causes this?

[Groily]

Not normal, no - but not that rare either.
Make sure that the brush touches the cb centre screw and that its head is clean. You should see a very small resistance if you put a meter from the head of that screw to the mag body - about half an ohm (which is the resistance of the low tension winding). If that's high, the screw head isn't squeaky clean and the contact may be weak enough to prevent proper operation.
Check also that the wire off is continuous to the button, with the button body properly earthed (at the 'bars probably). If, that is, things are assembled. You've already shown that the wire to brush connection seems good.
If all is good, then have a look at the main earth brush on the mag. It's under the big (usually-brass) screw at the drive end near the ID plate, easy to get at. If there's a poor (or no) earth there, then there isn't a good circuit back to the armature and the motor won't stop.
It will be one of these things, so should be a simple fix.

[DavidHulse]

Thanks for the advice Groily. I'll check it out as you suggest and report back.

[MikeM.]

I used to have problems with my K2F not cutting out and found it was due to oil on the earth brush next to the timing chest , dodgy seal. It didn't happen all the time, only when the oil built up and acted as a barrier. I use to take the brush out , degrease it and the mag and it was fine after that. Just a thought.

[SPRIDDLER]

It makes a change to have a post about how to stop an engine.

[DavidHulse]

Groily and Mike were right. I took the cb centre screw out and cleaned it but that didn't solve the problem. But taking out the earthing brush and cleaning it did the trick. Thanks for your help. Dave.

[G15 Roy]

Hope you are not planning on riding the bike far with an old Magneto as it's always a long push home.

[DavidHulse]

Don't worry, Roy. I'll definitely be testing it locally for a while before I venture further afield. The mag is original, but hasn't been run since 1972!!

[Groily]

Glad it came back to life Dave.

If there was enough oily crud to spoil the earth, I'm guessing the oil seal is hard as granite? They usually are after 50 years (!!) sitting around, not to mention its earlier years on the road.
You've got a faint chance that it'll be still OK when hot, but the HT winding is probably pretty tired, and without doubt the original condenser in there will have deteriorated.
It is definitely on borrowed time, through sheer age (regardless of usage).

As a absolute minimum, it is worth testing the HT side for continuity, as its absence says life is seeping away.
Fingers crossed, but as Roy says, all pushes home seem long, however short!

[clive]

Yes before Roy assisted me with the replacement of my mag windings I found the G15mk2 was quite a heavy push home and that was only about 1 mile and mostly flat. After standing unused for 30 or so years it had done 400 miles then the mag started playing up. I too was only going local but I am very pleased when it did not breakdown whilst I was at Burnham Beeches, its about a 17 mile push!