N1 Magneto

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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AJS_Chris
Posts: 28
Joined: Thu Mar 16, 2017 7:45 am
Location: Wiltshire

N1 Magneto

Post by AJS_Chris »

Morning all,

Having read many threads my '51 Model 18 fitted with an N1 magneto is exhibiting what I would say are 'classic' capacitor issues. The bike usually starts 2nd or 3rd kick when cold but after a long run can be a pig to start. I always carry a new plug as a spare and fitting the new plug does sometimes get the bike going, I put this down to the better spark being just enough to get it to fire. I've read the articles on 'condensectomy' and am quite happy to strip and work on the magneto myself (for me part of the pleasure of an old bike is that you can do these things). I know I could just ship it off but where's the fun in that?

However I do not know the history of the magneto so I am keen to prove that the magneto is at fault and have been thinking about how to prove this. My idea at the moment is to mount the magneto on my lathe and using a flexible coupling to the chuck spin the magneto at a fixed rpm similar to what would be expected when kicking the bike over. I plan to insert a screw into the plug cap and then progressively increase the gap from the screw to earth and establish the maximum gap the spark will jump when the magneto is cold. I'll then use my heat gun to warm the body of the magneto and repeat the test to see if the maximum gap is reduced.

Before I remove the magneto from the bike I'll go for a long ride and measure the temperature of the magneto when I get home, I'll warm the magneto to this temperature for the test. I have a Laser IR thermometer and will measure the temperature at the same point on the magneto both times.

At the end of all of this I hope to be able to say whether the magneto is at fault or whether I just need to learn how to start a hot Model 18. Apart from the 'why don't you just send it away' argument is there anything else I should consider?

One other puzzler. I changed the oil on the bike the other day draining the oil tank and removing the sump plug. I filled the bike with SAE40 and it stood for several days before starting while I did other routine jobs. When I came to start the bike the level in the oil tank went down but no oil was being returned, this went on for a few minutes until the bike stated smoking quite badly, normally there is little or no smoke at all. I stopped the bike and looked it over but found nothing. I started the bike again and instantly it was scavenging oil back to the tank at quite a rate and there was no smoke, presumably the contents of the sump were now returning to the oil tank. My only explanation is that there was air in the pump and the SAE40 that had been stood in the cold garage was too thick to be drawn into the pump, basically it wasn't primed. Once the oil warmed up a bit it was willing to be drawn into the pump and all was well. Is this a reasonable explanation?

Thank you in advance for any replies.
Chris.
Groily
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Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: N1 Magneto

Post by Groily »

Ref the mag Chris, no reason at all not to do it.
Most important thing is ensure the coil is good. If it looks quite recent it may well be - but you want to see somewhere around 5000 ohms across the HT spike or from the brass bit on the slipring to the armature spindle or poles. A coil will work with a small break, often for a long time, as the spark will jump it inside - but it's a one-way downhill slope! And you want it to stay solid to the squeeze when you warm it up - Lucas used 50°C as a standard test temp. It ought to make 5.5mm sparks by 130 rpm or less (fully advanced on the camplate) if you can get the lathe to run about that speed - 2 sparks a second roughly. A good one will do it from about 110rpm. I'd suggest you replace the slipring if it's an old one as they are often leaky when hot due to age. Also, I'd think about the HT pick-up, as the same thing can apply when things are good and hot.
Just flicking the mag back and forth over the flip point (the one where the points open) ought to make fat sparks too, easily able to jump 4mm or more. Good luck!
SPRIDDLER
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Joined: Wed Sep 01, 2004 1:00 am
Location: WEST SUSSEX UK

Re: N1 Magneto

Post by SPRIDDLER »

Wot Groily said, but................
AJS_Chris wrote: I always carry a new plug as a spare and fitting the new plug does sometimes get the bike going,
Is it possible that in the time it takes to change the plug the mag has cooled enough to start sparkling again............ :?

I had a similar issue in 34 degrees in France when my brand new Honda kept dying on me. I'd change the plug and it would go for around 5 mins then die. I'd changed the plug several times before it occurred to me that it might be fuel vaporisation. I removed the side fairings (left them in a bystander's shed) and it ran perfectly to Spain and back. It seems that in the time it took to change the plug th'engine had cooled enough to go again, for 5 mins.
'There is a tide in the affairs of men
Which taken at the flood............'
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Pharisee
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Re: N1 Magneto

Post by Pharisee »

With regard to your oil scavenging problem... I had exactly the same problem on my '55 M18S. Been for a ride, everything ok. Started it a few days later and there was no oil returning. Basically, it was pumping oil from the tank into the engine but it wasn't being returned.
It turned out to be loose screws (One tight, two loose and one stripped) securing the return side pump end plate causing air to be sucked into the pump instead of oil. In my case, that meant an engine out job to repair the stripped thread. Might be worth checking?
I'm from the Fens.... Gimme six.
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