classic motorcycle forum
Motorcycle Discussions => British Bikes => Topic started by: mark c on April 10, 2011, 09:02:22 PM
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Hello folks i was wondering if anyone had any suggestions about a irritating problem with my 1958 Huntmaster. It starts fine and if stopped restarts ok if cold but when it gets really warmed up then it is a pig to restart. If at that point i take the plugs out to check the spark then it aint there or is very weak . When it cools down the spark returns, looks ok then it restarts no problem. I have changed ht leads, cleaned and gapped the points, cleaned the thing that goes round in the magneto through the ht lead holes, checked the ignition timing and the tappets and it is still doing it. So far I haven't changed spark plugs, plug caps or ht lead brushes.
Any bright ideas appreciated
cheers
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It sounds as if the Magneto is in need of a rebuild/re-magnetizing for some strange reason the permanent magnets become less effective when hot which reduces the voltage.
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As well as tired old magnets, this also sounds like the classic Dud Condensor Causes Difficult Hot Starting syndrome.
Old condensors (buried inside the windings) weren't the best when new, and over the decades will likely have deteriorated - work OK, just, when cold, when hot are a no-go. Old wiring also deteriorates, as does the shellac doing the insulating - cold the spark still goes where it should, when hot goes awol.
A newly bebuilt magneto makes you realise that a new magneto is a wondrous trouble free device.
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Yes indeed, and if you want to both prove it's the capacitor failing and use a temporary solution to the problem, fit a new capacitor connected to the external "kill" terminal with the body of the capacitor to earth.
Normal sparks will be resumed... ;)
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Yes it could well be the capacitor which is at fault, but less likely, after a working life in electronics I have found that in most cases a capacitor which has lost some of its electrolyte will usually improve when heated.
Sometimes these old metal film capacitors start to go leaky reducing the available voltage and in effect have the characteristics of a resistor, this usually affects the spark hot or cold, but in any event I think we all agree the answer is a rebuild or replacement on the mag.
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Thanks for all the advice, i had wondered if the magneto was on its way out. I'll now show my ignorance i hadn't realised there was a capacitor/condensor [they are the same thing right??] on these bikes with a magneto! Do you have any suggestions where i can get the magneto fixed/exchanged/replaced?
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This would be my first choice for a refub, there are others just as good and many who are nowhere near as good.
www.davelindsley.co.uk/
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have found that in most cases a capacitor which has lost some of its electrolyte will usually improve when heated.
Not in old bike maggies it seems - impossible to start hot is a reasonably common fault in old maggies, and has been for a long time. [Assuming your spark plugs are the correct grade and not fouled or oiled].
Once cooled off, away you go again. I understood the heat allows the spark to jump to where it shouldn't, internally in the maggie, and the voltage to more easily escape in a dud condensor. (which are 2 different problems, it should be noted, but sometimes intertwined in old maggies)
Yes, condensors and capacitors are the same thing - buried inside the windings they are none too easy to replace. Since old wiring can have a green look to it, and the shellac insulation can suffer from 'shellacticus', indeed best to get it all done at the same time. You sometimes see NOS condensors avertised, which is ridiculous, they were none too good even at the time. And they don't improve with age.
Probably best you get your maggie redone (not sure if exchange unit are available these days anyway) since you have the core of a good maggie, and it is quite possibly quite original to your bike...
hth
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Not in old bike maggies it seems - impossible to start hot is a reasonably common fault in old maggies, and has been for a long time.
Agreed. I've never heard of a "hard to start when hot" mag to be anything other than a capacitor on the way out.... ;)
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As a professional electronics engineer with 50 years experience I find it quite amusing that amateurs know more about the subject than I do, and their knowledge is based on information in magazines written by other amateurs, quite amazing.
The capacitor in the magneto is placed across the points for the sole purpose of reducing the spark at the points to prevent them burning, if the capacitor becomes open circuit the points will burn, if the capacitor becomes leaky the spark voltage will be reduced regardless of whether the mag is hot or cold.
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Whatever symptoms or characteristics you feel a failing capacitor may exhibit, the fact remains that old mags fail over time, and engine heat exacerbates the failure (poor starting) symptoms; usually the mag will be rewound and a new capacitor fitted....problem solved.
But don't take my word for it, ask the many and varied magneto reconditioners out there just what the likely causes for a poor hot-starting mag, are, and how they'd fix them.
Hardly a new or unknown phenomenon in the old bike world...... :D
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Rex, I think you will find that is about what I said in my first post on this thread.
Your right, back to the shed for you :) :) :) :)
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Well, not just yet. Clarification needed first.
You said this-
It sounds as if the Magneto is in need of a rebuild/re-magnetizing for some strange reason the permanent magnets become less effective when hot which reduces the voltage.
then this-
Yes it could well be the capacitor which is at fault, but less likely, after a working life in electronics I have found that in most cases a capacitor which has lost some of its electrolyte will usually improve when heated.
which implies that you rather doubt the cap is the main culprit.
So, before I go back to the shed, which is your opinion? Cap or not? ???
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Wots a maggie?
Will it fit the heady on my compy Norty? will I need a new carbie for the potty? or a lampy? or will I weally need a wheely for my Trumpy motorbikies forkys?
Why cant australy speaky englishie properly?
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OK Rex,
A Magneto simple description, is either a moving coil or moving magnet, I'll assume in this case a moving coil, the coil consists of a primary coil and a secondary coil, the primary is wound on an iron former, this rotates through the magnetic field housed in the outer casing, as the iron former is just about to exit the magnetic field (the point of maximum flux), the points open as this happens the primary voltage increases to its maximum, this voltage it then vastly increased as it transfers to the secondary coil, the secondary coil feeds the spark plug via the slip ring and HT lead, the capacitor/ condenser suppresses the amount of spark at the points.
What can go wrong?.
1/ The magnets become less magnetized due to age, symptom, weak spark, even weaker when hot. (Will slowly get worse)
2/ The secondary coil has shorted turns, symptom, weak spark, same hot or cold. (Can continue running for years in this state)
3/ The secondary coil is broken due to vibration, symptom, none, the spark will jump the gap and slowly erode the wire ends until the gap becomes too big to jump. (Call the breakdown service)
4/ The Capacitor goes open circuit, Symptom, burnt points.
5/ The Capacitor becomes leaky, symptom, weak spark hot or cold, (will probably get worse until spark is lost)
6/ In the case of a manual advance unit, the cam stop wears allowing the points to open when the coil is not at the point of maximum flux, symptom, weak spark.
7/ On a face cam magneto, excessive end float on rotor, causes points to open late (after maximum flux) causing weak spark. quality of spark can vary with temperature change.
This is not a comprehensive list of faults available, but are the most common, any good re-builder will address all these problems.
The magnets whether in the case or when moving magnet need a keeper installed if the magneto is stripped for any length of time, so don't buy a replacement case from the autojumble it will almost certainly need remagnetizing.
In conclusion it would be a very foolish man who diagnosed what is causing the fault on a unit he has not examined.
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You clearly spent some time writing that, and although some may find it helpful, it wasn't what I asked.
I would disagree with some points anyway,namely-
4) the capacitor going o/c usually results in an extremely erratic spark, to the point that the engine all but refuses to run. The symptoms are much more extreme than cap failure on a coil/points engine system,
5) empirical evidence (both mine and a host of mag rebuilders) shows that the gradual deterioration in a mag's cap leads to hard starting when hot, and acceptable staring when cold. Clearly this will get worse until the mag becomes unuseable.
Also mags haven't needed (or used) a keeper since the days of Lucas KS/KN etc horseshoe magnet mags; after all, how would it be possible to place a keeper usefully or satisfactorily in a K2F or BTH, for instance?
It was just about giving the O/P some guidance on what is likely to be wrong with his mag; there's nothing definitive until it's inspected, as you say.
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I give up!!!!!.
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I give up!!!!!.
So you should.
In the old bike world, such tales are legion of hard-hot-starting magnetos from dud condensors.
Met a few myself........
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Wots a maggie?
You obviously have not met Miss Maggie Neato ?
"Ms Maggie Neato is seen at many events, promoting BSA motorcycles and related activities. While making the Norton look respectable, she's not slumming it ..."
www.bsa.asn.au/html/gallery/maggie/index.html
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I give up!!!!!.
What do you give up?
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The reasons for giving up.
First post I advise a rebuild/re-magnetizing, Its then suggested the cap may be faulty. I agree, even after I agree some still want to dispute what I say even though I was agreeing.
The original poster stated a total loss of spark when trying to restart when hot, this indicates weak magnets,
I list what are the most likely faults and advise re the magnets when stripped down, still people disagree.
Any engine which is receiving a weak spark will be difficult to start more-so when hot, most of the faults I listed will cause this problem including the capacitor.
Any magnet not working or fitted with a keeper will deteriorate, a magnetic rotor can be saved with a bit of old chain wrapped around it, magnets set in the body can be saved with a handful of nuts and bolts placed inside the housing.
I give up because it looks to me like whatever I say some just want to argue even when being agreed with.
What qualifies me to advise on magnetos?, when I left school in the dark ages I served an apprenticeship with Lucas refurbishing them amongst other things, I later moved on into electronics.
You can now all say whatever you wish about me, I won't be coming back to look.
Goodbye.
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Bet you already have..... :D :D
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..and indeed he did....just 24 hours later. ;)
Gotta love a man of principle.... :D
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Life is too short to worry about someone elses ailing magneto ?!!