Magneto

[Trevor68]

Hello all, I have had my magneto overhauled professionally and when bench tested gives a whopping blue spark but as soon as it is on the bike the spark becomes weak and when running the bike on the road it sputters and bangs in top gear, The timing has been set up and is bang on so not that, points are clean as well. So has anyone experienced this before and can anyone advise please. Best regards Trevor68

[SPRIDDLER]

Hello all, I have had my magneto overhauled professionally and when bench tested gives a whopping blue spark but as soon as it is on the bike the spark becomes weak

A couple of thoughts......

Did you have it overhauled because of the same symptoms that you have now?
Did you bench test it with the same HT lead and the same plug cap as when now fitted on the bike? If not, the current HT lead may not be making good contact with the mag slip ring. The plug cap must be the type without a resistor.
Is the mag body earthing well to its mounting platform? (I can't think how it wouldn't be, but....?)
Have you tried using a different sparkling plug? They can work fine resting on the cyl head in fresh air but can break down under compression.
Maybe when the mag is fitted the route of the HT lead is allowing the spark to jump to earth on the frame or engine.

[Ralph]

Think I would make sure the mag is earthed.

[Trevor68]

Hello all, I have had my magneto overhauled professionally and when bench tested gives a whopping blue spark but as soon as it is on the bike the spark becomes weak

A couple of thoughts......

Did you have it overhauled because of the same symptoms that you have now?
Did you bench test it with the same HT lead and the same plug cap as when now fitted on the bike? If not, the current HT lead may not be making good contact with the mag slip ring. The plug cap must be the type without a resistor.
Is the mag body earthing well to its mounting platform? (I can't think how it wouldn't be, but....?)
Have you tried using a different sparkling plug? They can work fine resting on the cyl head in fresh air but can break down under compression.
Maybe when the mag is fitted the route of the HT lead is allowing the spark to jump to earth on the frame or engine.

Thank you Spriddler I had it refurbed because of a weak spark but after fitting to the bike and timing set it still gave a weak spark so now I think after reading your answer I will be going through a process of elimination. I will report back sometime in the future with my findings. Best regards Trevor68

[SPRIDDLER]

I will be going through a process of elimination.

It seems pretty likely that there isn't a fault within the mag. It may help to know the year of the bike and which mag is fitted.
I've found that whilst looking for a fault by methodically doing the basic checks I've stumbled upon the fault and it wasn't one that I was expecting or particularly looking for.

As a couple of afterthoughts, if the HT lead is of the modern type (with a sort of compressed carbon dust core) it should be replaced with the stranded copper wire cored type.

I would also remove the plug cap from the HT lead and strip back the insulation on it enough to connect the wire's core directly to the plug.

You could try a test run with the plug gap closed to say 10 thou. It may seem pointless but it's worth a try.

You can check that the mag is earthed well by putting one probe of a test meter set to the Ohms scale on the mag body and the other onto a nearby shiny bit of the engine when you should get a reading of more or less zero Ohms resistance. If you don't have a meter connect a temporary test wire from the mag (say, from one of its mounting bolts) to a nut 'n bolt on the engine or the bike frame. However, it's unlikely that it is the problem or it would show up by misfiring at all revs.

As is the way with old bikes it's worth keeping an open mind as it may not be an ignition problem at all..........