Magneto no spark

[G3L1946]

Hi all,

My 1947 Matchless, recently acquired, has lost its spark! I did have it running, bit difficult to start, but start it did. When running it was fine. Started it, and ran it, four or five times. I was looking forward to my first ride out today ( in the sun ) but was unable to start it. Checked for spark……..NOTHING!
The magneto is a replaced and reconditioned one, the previous owner tells me ( and it certainly does appear to be so as it’s very clean and new looking!) which came from the club. However, this was several years ago and the machine has been dry stored for about ten years. Could it be something deteriorated during non use?
I have replaced and tested the HT lead. Cleaned and checked points and gap. Tried different spark plugs. Is there any other tests I can carry out to find the fault? I will remove the pick up assembly and take a look at that.
Before I start removing the magneto I want to test as much, in situ, as I can.

Any advice, much appreciated.

Steve

[Groily]

It's not unusual to be sparkless after hibernation owing to oxidised points, but it is a bit less typical for the thing to have started several times and run OK, then decided not to play.
Presumably this is a standard N1 magneto.

The things you can do with it on the machine are limited. You've done the obvious, with HT lead & plug, points etc. Provided the gap is about right and that the sprung (live) contact breaker point isn't touching either the inside of the cover (it happens!), or the head of the screw underneath its tongue which holds the parts of the cb unit together (both of which which will cause a short to earth) all should be OK in that department.

It would be worth putting a meter between the inside of the plug cap and the mag body to check that there is a resistance reading of say 5000 ohms, which represents the resistance of the HT winding. This would prove continuity from plug cap to cable to HT pick-up to brush to slipring to coil to armature to earth brush. Similarly, if you take the HT pick-up off and measure from the brass track of the slipring the brush runs on to the mag body, you should see the same resistance. If there is no reading in the former case but is in the latter, the problem is between the pick-up and plug; if the latter, then it's in the mag.The mag body needs to make a good earth to the mounting / frame / engine as well, for there to be a return path for the HT current.
Worth checking, if there's an apparent problem with continuity, that the earth brush is OK and making good contact - it's hidden under the magneto's id label.

Lack of HT continuity spells trouble because unless it's 'just' the earth brush, there's a problem on the armature, but I'm guessing, because it has run properly already since its hibernation, that the reason for a sudden total lack of sparks is on the low tension side, which leads me back to the contact breaker area. But can't rule anything out I suppose . . .

Also need to check that the camplate isn't out of position, with the advance / retard cable having jumped out of its slot and jammed things up with the plate in the wrong place. That also happens, but you'd notice something was weird because the cable mechanism would be stuck. And if you've had the points assembly off, check the pushrod that opens them is seating properly in its recess and is free to move easily.

[G3L1946]

Hi Groily,

We speak again! Thanks for getting back to me with your comprehensive reply. I’ve downloaded the Lucas service sheet for the N1 magneto and, together with your suggestions, will have another look at it today. On the face of it, and inspection, the points side of things look O.K although I did notice a bit of rusting around the points lock nut area. I’ll probably end up taking it all apart and giving it all another thorough cleaning. As above,I will take the pick up assembly off to see if any issues lie within.

Thanks again,

Steve

[Groily]

I'd be surprised if there's anything too much wrong with it Steve to be honest, but as we all know they can lead us a merry dance sometimes. Even rebuilt ones occasionally!

[G3L1946]

So true!

[G3L1946]

Hi Groily and all,

As suggested, I tested the resistance between the spark plug cap and body of the magneto. Nil! No reading. So, took off contact assembly and found a ‘temperamental’ continuity between the contact pin and cap end of HT cable. I sprayed the contact pin with electrical contact cleaner and carefully cleaned with a paper towel. Muck! Also, sprayed inside contact assembly. Re fitted, retested and got a reading of 4850 ohms.
Did a test to see if spark at plug and there it was.
Just going to have a coffee then will wheel her out for final test. Will it start!
Thanks Groily!

Steve

[Groily]

Sounds good Steve. That's your 5000 in round numbers, which is what I'd expect to see on a rewound coil from most of the usual suspects. Fingers crossed she goes now. Not wanting to tempt fate of course, as it's just round the corner with brick in hand as ever . . .

[G3L1946]

Isn’t it just!
Cheers