Author Topic: Magneto capacitor  (Read 541 times)

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Offline yosemite

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Magneto capacitor
« on: 24 Jan 2020 at 22:24 »
The T35 is giving problems when its warmed up occasional misfiring and virtually impossible to start ,  when its cold the bike starts and runs well, so  I have come to th concluion the likely cause is a failed capacitor, Iwas told that the mag had been rebuilt shortly before I got the bike three years ago but am not sure exactly what work was carried out. I've been thinking about stripping the magneto  and armature but after lots of reading on the web have become confused over 1 point. that is wether or not I need to apply keepers to the magnets? Several sources say I should but a lucas manual, dated 1953 says I do not need to. As the magneto was built by lucas I assume this is correct, but why do so many say I need to. Is it because as I assume  lucas used a different material for their magnets to other manufacturers ?
Thanks
Paul

Offline EW-Ron

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Re: Magneto capacitor
« Reply #1 on: 25 Jan 2020 at 04:30 »
There is quite a good discussion of the technicalities of this here
http://www.brightsparkmagnetos.com/faqs/FAQs%20about%20magnetism%20and%20remagnetising/What%20is%20a%20keeper,%20and%20do%20I%20need%20to%20use%20one%20if%20I%20take%20the%20armature%20out%20of%20my%20rotating-coil%20magneto.htm

With those numbers quoted, and I've a small collection of old magneto bodies of various varieties, and judging by the lack of residual magnetism in some of them,
a keeper would appear to be an essential part of any magneto work or refurbing of them  ??

P.S. Something you may like to contemplate is the range of 'easycap' substitute capacitors supplied for various brands of magneto - without knowing what your Douglas would actually use.
These are simple to install - much simpler than extracting the buried condensor in many a magneto.
http://www.brightsparkmagnetos.com/easycap/index.htm
« Last Edit: 25 Jan 2020 at 04:38 by EW-Ron »

Offline Doug

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Re: Magneto capacitor
« Reply #2 on: 25 Jan 2020 at 06:02 »
Paul,

Given the rebuilt status of the magneto, another possibility is a defective slip ring. There was a batch of these made of the wrong material some years ago which was hydroscopic. When the magneto warmed up, these would sweat moisture and arc, causing misfires and heaven help you if the engine stopped you would never get it restarted until the surface dried out. One would think all of these have been found or eliminated by now, but you never know.

I though generally when capacitors go bad, they pretty much terminally die?

-Doug

Offline EW-Ron

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Re: Magneto capacitor
« Reply #3 on: 25 Jan 2020 at 08:08 »
Difficult to start hot is classic condensor problems - and it starts oh so easy when cold.

I've had several magdynos (not Douglas) where this rapidly got progressively worse, to the point where you
dread stopping it.  Once experienced, you get your magnetos in tip-top condition, and keep them like that !
And keep a spare on the shelf, for the 1st sign of trouble.
Haven't actually tried those BrightStart little thingies, but they seem like an answer to a prayer.

Hearing that the old foil type NOS condensors could be on their last legs new out of the box just adds to this
- and yet you still see them for sale ...

Online eddie

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Re: Magneto capacitor
« Reply #4 on: 25 Jan 2020 at 09:59 »
Evidently, with those 'Brightstart thingies' you still have to dismantle the magneto in order to take the original condenser out of circuit. If your original condenser is just getting weak, you could try adding another condenser between the cutout wire and earth to see if there is any improvement. One for a Morris Minor is usually easily available! Other than that, send the armature off to APL Magnetos in Shaftesbury, Dorset for a rewind (usually about 100) - I have entrusted them with the rewinds on my competition magnetos and have had no problems since.

  Regards,
                 Eddie.

Offline EW-Ron

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Re: Magneto capacitor
« Reply #5 on: 25 Jan 2020 at 20:47 »
You only have to remove the armature and snip the connection to the old foil type condensor.

Which often fail, way before the wire needs a rewind.
Although its good insurance to get the whole lot done in one go, its a heck of a lot simpler
(and cheaper ) to just add one of those BrightSpark thingies. ?

Lots of satisfied customers it seems - I must get around to trying one of these dodgy magdynos I have with one.
They start so easy cold that there can't be much wrong with the wire in the windings... ??

Offline yosemite

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Re: Magneto capacitor
« Reply #6 on: 26 Jan 2020 at 10:08 »
Think that Iwill try to put an extra in circuit as eddie has suggested, I have a couple of spares for the A65,  if that cures the problem then may  down the brightsapark route.
just had a thought, if the condesor inside the mag is dying is there any reason I cant disconnect it and use an external one permanently? that way if I get further problems it will be easily replaceable.

Offline dalgrae

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Re: Magneto capacitor
« Reply #7 on: 26 Jan 2020 at 14:04 »
Hi I have had a Britespark fitted to my mk5 magneto during overhaul ant had no problems at all,the britespark web site video clearly shows how to fit one yourself ,but it was too daunting for me

Colin

Offline EW-Ron

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Re: Magneto capacitor
« Reply #8 on: 26 Jan 2020 at 19:40 »
is there any reason I cant disconnect it and use an external one permanently? that way if I get further problems it will be easily replaceable.

What sort of magneto do you have ?

The main difficulty of having an external capacitor is that most common British magnetos of the era had a rotating coil,
which means the external connection to the cap has to be through something like the earthing system.
These are often very light duty so performance may be a bit iffy, especially long term.
I think I saw that the BrightSpark website (above) discusses this along the way ?

Offline yosemite

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Re: Magneto capacitor
« Reply #9 on: 26 Jan 2020 at 20:51 »
Went out on the bike with an extra condesorbetween the cut out wire and earth as suggested by eddie, did about 20 miles though never went far from home, bike ran fairly well apart from occassional infrequent missfire, didn't stop engine till I got home, then bike restarted easily  so stopped and disconneted extra capacitor and it was impossible to start so resconnected capacitor and bike started easily, so it definatly seems to be capacitorwill leave on for now until I decide what route to go down, either a rebuild or an easy cap condensor, probably the latter. just for cost.
think the mag is a lucas mn2 never paid much attention to it apart from drying points out not long after bI got it andchecking timing.
thanks to all for your help

Offline dalgrae

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Re: Magneto capacitor
« Reply #10 on: 27 Jan 2020 at 07:09 »
Hi I have an mn2

Colin

Offline yosemite

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Re: Magneto capacitor
« Reply #11 on: 27 Jan 2020 at 08:59 »
Hi
ordered a brights park easycap will fit that when I get it  hopefully it may get here before brexit, may look for a spare mag depending on funds
Colin sent you a pm
THanks
Paul

 

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