Effect of magnetizing a KVF


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This post will be less detailed than I ordinarily like to write, but I thought it would be helpful to put some actual numbers out for examination. I don't really won't have time for another two weeks to be doing this, but I made some time this evening anyway. I had never measured a KVF in the way described below, but please see the post on magnetizing the Bosch ZEV and the sidebar on magneto testers at the following link on Britbike Forum for the necessary background:


The magneto I tested tonight (date code 4/49) had been rebuilt ~15 years ago by the same guy who used an inappropriate condenser in my Gold Star's magneto, whose failure after ~50 miles resulted in me setting out to do this work myself. I will be rebuilding this magneto later in the process of restoring my Black Shadown in the days (weeks, months, years... to come). I left the cover off the points cavity while running the following tests to make sure the condenser was still functional (there are 0 miles on it).

I magnetized my KVF tonight using my magneto charger described in the above thread, then installed it on my lathe and ran the same "low speed test" as described at:


I had the leads attached to separate 6 mm gaps on my Merc-o-Tronic test board. I did not use proper 3-point gaps for tonight's tests, so the values I found won't be identical to those from proper gaps. However, the relative values all will be just as I determined them this way. Actually, I'm not going to fully describe all the experimental conditions because I need to get this post uploaded because I have other things I have to get done tonight.

Anyway, with the magneto freshly magnetized in the fully assembled state, one lead fired reproducibly down to ~160-170 rpm (320-340 rpm engine) and the other to ~190-205 rpm (380-410 rpm engine). OK, another note: these are the rpms from the dial on my variable-speed lathe. They are somewhat close to the actual values, but I would have had to take the time to calibrate with a strobe, which I didn't do tonight. I ran the lathe up and down from ~500 rpm several times, each time finding the same low speed behavior. Anyway, what this shows is that the Vincent engine would have to be kicked over at ~400 rpm for both cylinders to fire with this magneto. I should add that even the lower of the two speeds is somewhat higher than I find with my rebuilt K2F magnetos.

I then removed the armature from the magneto, left it outside for maybe 10 sec., and then reassembled the magneto. When I re-ran the tests I found one lead fired reproducibly down to ~200-205 rpm (400-410 rpm engine) and the other to 230-250 rpm (460-500 rpm). What this shows is that if this particular magneto were rebuilt without being remagnetized, or if it were magnetized while disassembled, it would work fine. Other than the kickover speed being ~15-20% higher after the rebuild.

Again, the guy who rebuilt my KVF screwed up my Gold Star's magneto, so I will be rebuilding this one myself. Also, the rpms given above are from the lathe's dial and so are only roughly correct. Also, a variety of members of the alnico family were used during the 1950s, with slightly different B-H curves, so the precise loss of power will vary somewhat depending on date of manufacture. However, despite these and other qualifications, the ~15-20% loss in performance I found should apply to all. That said, if you have no problem starting your Vincent, none of this matters.

Depending on what kind of interest there is in this topic, I'll post more on this in the the future. But, it will be a few weeks before I can add much.


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Please let us know ,who not to send our mags to here in the states , PM me if you need.
The guy who "restored" my two magnetos 15 years ago is no longer of this earth so he's out of the picture.

I would love to be able to confidently recommend someone(s) who I knew would do a proper job of rebuilding and remagnetizing magnetos. Unfortunately, the only way I could do that would be if I had seen their work for myself. The "problem" is that good restorations don't fail, but a lot of bad ones soldier along longer than one might expect, so just the fact a magneto works for a while isn't evidence of a good restoration. Since I've only seen the "quality" of the work on ones that have failed (and, in only a few cases am I completely certain of the person who did the work), I don't feel comfortable making either an "approved" or a "disapproved" list. Instead, what I've tried to do with the detailed magneto restoration on Britbike Forum is give people the information they need to identify for themself restorers who will at least use proper components and magnetizers. I go into some detail on this in the Epilog to that thread: