M01 magneto

[Greenfield]

I was out on a test run of around 5 miles on the B33 last week checking for leaks after rebushing the outer gearbox case. Less than 1/4 mile from home the engine died and wouldn't start. Luckily I could coast home downhill. There is no spark at the plug or HT lead. I have checked and cleaned the magneto brushes, but still no spark. Before I get the magneto onto the bench, am I missing something obvious? A previous owner spent a bit of money on a full service including an exchange armature in 2014 and the bike's done little mileage since. Strangely, the armature was replaced again after a month as the replaced one failed!
Any thoughts chaps?

[chaz]

fuse?
connector?
earth?

thats where I would start before stripping the mag.

[Greenfield]

Agreed, always start with the obvious.
There's no fuse or connectors in the magneto circuit. When I got the bike nearly two years ago, I chatted to the magneto guys re the earth terminal which had no lead fitted. They felt that it shouldn't be important as it should earth through the frame, but when I rewired the bike, I added an earth. Up until last week it always started and ran well.

[Bess]

Hi,
     I found this place a good resource, link here:

http://brightsparkmagnetos.com/DIY/index.htm

Best wishes...

[idie]

An obvious thing. Is the magneto armature rotating. Make sure that the points spring is not touching the cam on rotation,if the little screw backs off the spring will move on its slot. Check that the commutator is not contaminated with carbon, there is some soft brushes around. Is the little brush still in the black pickup.

[Greenfield]

This'll keep me busy for a while. I'll report back.
Cheers

[Pete C]

Also check the points are actually opening and the HT pick up hasn't worked loose (there is no commutator, that's in the dyno part)
Could also be a duff plug, try getting a spark direct from the HT to earth, or a different plug.

good luck

[idie]

Must put brain in gear. Confusing mag with dynamo.

[JulianS]

The mag part has a slip ring which can become contaminated with carbon from the HT pickup brush.

[Greenfield]

Ok so here are my preliminary findings:
I first of all looked at the HT circuit from slip ring to plug cap and everything looked good
My next test was to check the circuit across the points - there was continuity whether the points were open or closed
Resistance test between armature and body (earth brush removed) - gave a reading of only about 1.5 ohms
My thoughts are that with little resistance across the magneto, the bearing insulator is no longer effective, and a strip down is needed to explore things further.
Happy days

[Bess]

Hi,
    Priory magnetos would be my go to, link here:

https://sites.google.com/site/priorymagnetos/services


Best wishes...

[Derek996]

Ok so here are my preliminary findings:
I first of all looked at the HT circuit from slip ring to plug cap and everything looked good
My next test was to check the circuit across the points - there was continuity whether the points were open or closed
Resistance test between armature and body (earth brush removed) - gave a reading of only about 1.5 ohms
My thoughts are that with little resistance across the magneto, the bearing insulator is no longer effective, and a strip down is needed to explore things further.
Happy days

The armature primary resistance is quite low, around 0.5Ω, so difficult to measure and you will see little difference in resistance measuring across the points with a digital multimeter between points closed and points open. The armature should earth through the earthing brush, located on the side of the magneto. Check that it is intact and making contact with the armature. The bearing insulator is to prevent the armature earthing through the bearing as sparks could damage the bearing surfaces.

[Greenfield]

Thanks Derek. This is unknown territory for me, so I've been working through the 'Brightsparkmagnetos' check list provided by Bess. It says with the earth brush removed and assuming I have a fibre pinion drive, the body to armature shaft resistance should be over 500 ohms. I have around 1.5 ohms. Hence my thoughts re bearing insulators. Superficially everything looks good, but the magneto suddenly failed with no warning after being totally reliable.

[idie]

My mag on the B33 stopped working some years ago and this turned out to be the bearing insulator. I think this started after the mag got soaked with rain water. Although the mag dried and worked I think the insulator broke up when it got wet.

[Pete C]

I didn't think the bearing insulators had any effect on the functioning of the mag except to protect the bearings, as stated by Derek. You should still get a spark even if the insulators are toast.