Rebuild on my A10 now nearing completion and I've just finished all the wiring. Engine is from a 1955 Golden Flash with a Lucas K2F magneto. Problem is occurring with the magneto kill switch wiring in that it is permanently earthed, which as I understand it will stop the engine running. Traced the fault through the wiring and confirmed they all switches are working properly. With the kill switch wire removed from the magneto the wiring problem goes away but the magneto itself appears to be earthing out. There is a direct connection between the earth electrode and the magneto body which doesn't sound right. When I assembled the bike I checked the magneto and it was producing sparks so my assumption was that it was ok. The bike was bought as a "box of bits" so I don't know when things on it last worked.
My knowledge of magnetos is very limited and even after reading the Lucas workshop instructions I am none the wiser. It may be they there is nothing wrong but my view is that if it is earthing out it won't start.
I would appreciate any advice anybody might be able to give with regard to the problem or whether I am worrying about nothing. The bike is not quite ready for starting and I would rather sort any problems before firing it up.
Your message is a bit confusing. By earthing brush are you referring to the one in the mag body near to the timing cover or the one in the cap on the end. If you remove the cap does the fault disappear, if so then the fault is in the cap. The brush in the cap bears on to the points unit and is insulated from earth until earthed out by the switch or button. The other brush is an earthing brush so don't remove that or you could damage the armature.
Thanks for the response. Sorry if the original message was a bit confusing. What I was trying to say was that the fault does not appear to be in the wiring or switches but in the magneto itself. The earth is the one in the end cap. The brush in the end cap sits on the head of the bolt in the centre of the points. I have removed the end cap and checked the continuity between the bolt in the centre of the points and the magneto body and this is showing direct continuity between the two.
What's confusing me is that the magneto does produce sparks which I wouldn't expect if it is earthing out. As I don't know much about the inner workings of a magneto I was hoping someone with more knowledge could suggest possible causes and hopefully fixes.
Probably nothing wron with your mag.
There should be a circuit between the centre screw and the mag/armature body.
Easiest way to see it is the page from the Lucas workshop instructions below and primary winding continuity test.
Thanks Julian. That's the same workshop instructions that I have already got.
As you say it may be that the mag is ok but I am still confused.
If the centre bolt forms a circuit with the mag body and by bolted connections the mag body is earthed through the engine cases and frame, then there must be a continuous circuit all the time. Why does operating the kill switch to earth what is essentially the same circuit have any effect?
I've no doubt there is simple answer and it's only because of my ignorance that I'm finding a problem which maybe doesn't exist!
I'll carry on and see if the engine will fire up when ready in the next couple of weeks but it would be nice if I actually understood how it all works
I'm no expert on magneto's but my understanding is that one end of the primary coil is effectively earthed to the magneto body internally and the other end is connected to the contact breaker (CB) centre screw. When the contacts are closed, both ends of the primary coil are therefore effectively earthed to the magneto body. When the points open, current will start to flow through the primary coil and via the secondary coil then produce the HT spark.
With the points open, the cut-out brush should contact the CB centre screw and connect to the magneto body via the resistance of the primary coil. When the points are closed, there should be zero resistance to the magneto body. If this is correct, the cut-out terminal should connect to earth (magneto body) only when the contacts are closed but should otherwise show the primary resistance of ~0.5 Ohm? For reasons I probably don't understand, it might be better to perform the continuity test as recommended by Lucas using a battery and ammeter rather than using a modern multimeter sets on Ohms??
Thanks AJWD. Just checked it again and set my multimeter to a much more sensitive setting. With the points open it is actually reading about 0.4 ohms. With all wires reconnected this is the same reading as I get from the kill switch wiring. When the kill switch is operated the reading falls to zero as you would expect.
As I suspected it was my ignorance making problems where there weren't any. Hopefully all will be ok in a couple of weeks when I fire the engine up - that'll be the real test
I guess the proof is in the pudding. If you get a spark and the engine runs ok and the engine stops when you press the cut-off switch then everything is working as it should! Coil ignition is definitely easier to grasp!!