Magneto Magnet strength ?

[SEDoan]

Found my notes on the performance before and after recharging the magnet.

Before recharging, magnetic resistance to rotation was very weak – barely noticeable. Using a homemade 3 point spark tester with a 5mm gap the mag sparked about 2/3 the time at 130 rpm. Unfortunately I forgot to note if it was retarded, fully advanced or in between.

After recharging the mag had much more rotational resistance and spark would jump a 5mm gap every time at 130 rpm retarded or advanced. At 80 rpm (the next slowest speed on my old lathe) with timing fully advanced, the mag missed 5 times in 60 seconds – sparking about 94% of the time at 80 rpm.

I also kept score after installing it on the Ariel and over a few days of riding it started 20 times with 24 kicks and never required more than 2 kicks to start. Since then the score is note quite as good, but that may be me misusing the choke...

We recharged 2 mags a friend has and we agreed that the magnetic resistance was greater on both and the sparking was better on the one that sparked. We did not have time to measure before and after.

[nevhunter]

Without a safety gap the coil may discharge through itself and do damage to the coil. This can happen if a plug lead falls off when youare motoring along the road. IF it doesn't spark at the plug it has to go somewhere.. Nev

[Simon.Gardiner]

....no matter what has been done to it, if the armature has been out of the housing, it has to be magnetized.

However, one of the points I took from Charles' earlier link to the britbike forum was that the 1950s magnetos don't need a 'keeper'. Having fretted quite a lot about leaving MO1L and K2F bodies lying around without the armatures in them I think this needs emphasising.
I'll repeat the link here with and quote the text:
https://www.britbike.com/forums/ubbthre ... Post460999
"The Lucas N1, KN1, K1F, K2F and KVF magnetos and MO1 and MN2 magdynos include internal keepers (the extensions of the pole laminations that extend underneath the armature and almost meet opposite the magnet). That is presumably why Lucas explain in their 1953 workshop instructions specific to these models of magneto that it is not necessary to apply a keeper when removing the armature. Equally, an extra keeper is not required when removing an internal remagnetising core from and inserting the armature into these particular models."

SG

[cmfalco]

However, one of the points I took from Charles' earlier link to the britbike forum was that the 1950s magnetos don't need a 'keeper'.

No, you took away incorrect information. The post you quoted was written by an attorney, not by me, who co-owned the company that sells replacement capacitors that fit in the end housing after removing the armature and snipping the leads to the malfunctioning original capacitor. It was in his interest to say remagnetization wasn't necessary. However, my measurements show while it isnt' "necessary," because it still will be possible to kick start a motorcycle afterwards (unlike the case with pre-Alnico magnetos), it will require a significantly higher kick starting speed because the internal "keeper" only partially reduces the loss in magnetism.

[Simon.Gardiner]

Charles
To be clear on this tricky subject, I'm not saying I saw that re-magnetisation was un-necessary; what I saw was that the later magnetos don't suffer the same dire consequences of having an armature removed that the earlier ones do.
Which salves my conscience about leaving some MO1L and K2F bodies in one place on the shelf with the armatures neatly stacked next to them.
(I've definitely got one MO1L body with not a lot of 'pull' - but it starts the bike well enough so in line with your observations about low magnetism and low speed performance I'm also much happier about that one now.)
SG

[nevhunter]

I would have thought that any kind of Keeper would reduce the magnetic field strength left remaining to go through the armature. When you are magnetising with the armature in place, the armature diverts some of the magnetic flux from going through the Magnet. Nev

[cmfalco]

When you are magnetising with the armature in place, the armature diverts some of the magnetic flux from going through the Magnet.

Although that's true, if it weren't the case, a smaller electromagnet could be used. Also, as the armature rotates, the magnetic flux changes. The discussion would take too much background information for a proper explanation, but maybe the following simplified figure will suffice.

Without explaining what 'B' or 'H' are, the magnetization follows a BH hysteresis loop in operation. When the magnet is fully magnetized it oscillates back and forth along something like the red line at the top as the armature rotates and diverts some of the magnetization during the oscillation cycle. However, if the magnetization has been reduced by removing the armature, once it is replaced the magentization will have dropped to something like the lower red line, and will oscillate back and forth along that lower line as the armature rotates.

The nature of the hysteresis loop, which depends on the material properties of the magnet, has to be understood for an actual explanation, but chapters of books are devoted to providing the explanation so it is well beyond what can be summarized here in more than a superficial way.

[nevhunter]

Yes true. I've at times tried to hold the armature where it gives the LEAST path. The general view with Mr Lucas seems to be to have a strong enough magnetiser and all will be well. .. Nev