Author Topic: A10 K2F Magneto question  (Read 2833 times)

Offline Lannis

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A10 K2F Magneto question
« on: 30.07. 2009 04:01 »
I obviously don?t understand how a magneto works.

Here?s what I?m into.  The A10 with a K2F magneto runs great.  Starts first kick every time, idles and runs well.  Runs so well that it will no longer shut off with the handlebar button kill switch.  It used to, but gradually started NOT working; I?d press it, and it would start running on one cylinder, and gradually die.   Last week, it quit working altogether, and I had to stall the bike against the brake to kill it.

So I disconnected the kill wire at the magneto and tested the button.  Button OUT, no ground at the spade connector.  Button DOWN, it connects to ground.  So the button works.

I then thought the brush inside the end cap must be not making contact with the points center bolt.   So I screw off the end cap and put a new brush in.

But meanwhile, I connect one lead of my ohmmeter to the engine case (ground) and test the magneto with the other lead. 

Any place I touch on the magneto has continuity to ground.   The center bolt is grounded whether the points are open or closed.  Each point has the same continuity to ground, whether the points are open or closed.

And yet the bike runs, and runs well.  Of course the kill button doesn?t work; the center bolt on the magneto is ALREADY at ground, all the time.

What don?t I understand here?   You may call me Jack-pudding if you like and I will take no offence, as long as I can solve this silly thing.


1961 A10 Golden Flash
1969 A65 Firebird Scrambler
1955 M21 Commodore
1935 Matchless Model X Project
1990 Moto Guzzi California III
1983 Moto Guzzi 1000SP
1986 Yamaha TT225 trail bike
1966 Morgan 4/4

Online muskrat

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Re: A10 K2F Magneto question
« Reply #1 on: 30.07. 2009 04:45 »
G'day Lannis, The earth brush is faulty. At the end just in from the flange you will see a screw. Under it is the earth brush & spring. This is what makes earth with the magy. I think from memory the carbon center from a "C" size battery can be made to replace it (Quick Fix). Not a bad problem except when carbs stick full open.
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Re: A10 K2F Magneto question
« Reply #2 on: 30.07. 2009 07:59 »
Sorry if you've read this before somewhere, but...
 The centre bolt etc., of the magneto is not short circuited to earth with open points. There is resistance through the primary windings of the mag, but that resistance is too low for you to see on your meter.

With the end cover off the mag and the cutout lead connected to its little stud in the cover, test for continuity between brush and earth with your meter. There should be none until you press the kill button. If the kill button does not connect the brush to earth, you have a simple continuity problem in the circuit: brush, spring, holder, terminal, lead, button, frame, magneto casing.

Then make sure the cutout carbon brush is touching the head of the bolt- you should be able to feel the spring before the screw threads of the end cap meet as you install the cap. I'm shocked at you just buying a new brush without stretching the spring first!!

Are you keeping an eye on your points gap in case the pillar come loose again?