1931 VB magneto

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Kevin.Priest
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1931 VB magneto

Postby Kevin.Priest » Mon Jul 09, 2018 7:23 pm

Trying to understand what’s going on in my mag, any advice welcome .
The points when open have continuity between the contacts which can’t be right . If I slacken the central bolt by 1 turn then I have open an circuit . So how is this supposed to be insulated ?
Picture is of the central bolt removed and laid in so you know which bolt I am referring to .
Hope you can shed some light to help me understand .
Cheers in advance
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simon.holyfield
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Re: 1931 VB magneto

Postby simon.holyfield » Mon Jul 09, 2018 7:54 pm

Are you sure you have true continuity, and not the (very low) resistance of the primary winding?
cheers

Simes

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alan.moore
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Re: 1931 VB magneto

Postby alan.moore » Mon Jul 09, 2018 8:09 pm

This is normal with a magneto. As Simon says, what you are seeing is the effect of the low resistance of the primary coil.

The points are wired in parallel with the condenser and the primary coil. When the points open any electrical current you are using to test for continuity (battery and bulb) will find a path to earth via the primary coil. If you try to use an ohm meter the primary coil has a very low resistance (about 0.5 ohms) and the meter cannot differentiate between points open or close. Removing the centre screw breaks the circuit to the primary coil and that is why you can then see when the points open (open circuit).

The magneto 'buzz boxes' you can buy measure the impedance of the circuit (as opposed to resistance) and are therefore able to differentiate between the two different states.

You can also use a small (3 volt) electrical buzzer powered by two 1.5 volt batteries , the tone of the buzzer will change when the points open.

Cheers
Alan
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Kevin.Priest
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Re: 1931 VB magneto

Postby Kevin.Priest » Mon Jul 09, 2018 8:14 pm

Cheers Simon and Alan I may have other probs then . Need to get this sorted as it’s off to German Rally in sept and the Mag guys seem to be very busy .

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alan.moore
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Re: 1931 VB magneto

Postby alan.moore » Mon Jul 09, 2018 8:52 pm

I take it you have tried Tony Cooper in Birmingham?
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Kevin.Priest
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Re: 1931 VB magneto

Postby Kevin.Priest » Mon Jul 09, 2018 9:49 pm

I think Tony is going to be the best as he’s working on 4-5 weeks at present . I’ll get it off to him tomorrow and keep my fingers crossed .i better get another bike ready just in case .

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Re: 1931 VB magneto

Postby nevhunter » Tue Jul 10, 2018 1:28 am

Remove the rotating points assembly and check with it off. There's sometimes a short circuit due incorrect washers. Do not put a largish current onto the centre screw as it may affect the magnetism in the permanent magnet. (ask me how I know). Nev

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Re: 1931 VB magneto

Postby bevanclark » Tue Jul 10, 2018 7:07 pm

Just curious - in Kevion's photo what is the little points opening block made of and are they obtainable ?
Cheers, Bevan

Kevin.Priest
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Re: 1931 VB magneto

Postby Kevin.Priest » Tue Jul 10, 2018 7:55 pm

nevhunter wrote:Remove the rotating points assembly and check with it off. There's sometimes a short circuit due incorrect washers. Do not put a largish current onto the centre screw as it may affect the magnetism in the permanent magnet. (ask me how I know). Nev


Sorry Nev you asked , tell me how do you know ?

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Re: 1931 VB magneto

Postby nevhunter » Wed Jul 11, 2018 12:08 am

Because I used a battery to cause a spark on twin magnetos, with a piece of cardboard to hold the points open and the magnetism virtually disappeared in the permanent magnet. The purpose was to check and confirm which pickup was sparking when the timing was done. (Important on a twin) I had to remove the maggy and remagnetise it in the normal way. ( I have a professional magnetizer. 2 people can hardly lift it.. You usually have to remagnetise a magneto if you dismantle it or if it's been sitting or bumped around).
Just a comment to the image. There's a small non conducting button on the end of the pivot shaft the "L" shaped point moves on .It's missing. The clip sits on it. It just keeps the thing in place. won't stop it running but better its there.
To Bevan, the contacting block is made of the usual electric stuff like what older radio valve sockets were made of. It's like a canvas layered bakelite. You can buy blocks of it and cut, drill or mill it. I make insulating terminals and washers out of it regularly . Nev


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