Magneto Coils and Condensers
John Lawrie
Villiers Mk20 engine with flywheel magneto, with no spark, and a
Wolseley WD1 with a weak spark. I assume the problem may be an open
circuit within the coil, however I am unable to confirm this.
Any suggestions would be welcome.
Regards
John Lawrie.
--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG
Rob Skinner
<john.l...@btinternet.com> wrote:
>Can anyone suggest a Method of checking coils and condensers. I have a
>Villiers Mk20 engine with flywheel magneto, with no spark, and a
>Wolseley WD1 with a weak spark. I assume the problem may be an open
>circuit within the coil, however I am unable to confirm this.
>Any suggestions would be welcome.
Hi John,
I'm not familiar with the engines that you're asking about, but a
mag's a mag, right?
Testing coils is pretty easy. If you have the coil on the bench, put
an ohmmeter across the primary (the circuit with the points). You
should read a low resistance, say about five ohms. It could vary by a
few ohms, but since the wire is relatively large guage, it's probably
ok. If the readings are in the ballpark, don't worry about it.
Now put the meter on the secondary (the circuit with the spark plug).
You'll read a lot higher resistance, probably between 5,000 and 10,000
ohms. Again, since we don't know exactly what the resistance should
be, if you're in the ball park, consider it ok.
There is a caveat to checking the coils with a multimeter. Under
real-life conditions, the primary side will be experiencing hundreds
of volts and the secondary side will be getting tens of thousands of
volts. Your multimeter probably checks it with about nine volts. If
the insulation on the windings is marginal, it may check out ok at
nine volts, but break down at higher voltage. In other words, your
test can prove that a coil is bad, but it cannot prove that the coil
is good.
The same applies to the condensor. I've found capacitance meters to
be virtually useless for working on mags. Sure, you can prove that
the condensor is bad, but I can't remember EVER finding one THAT bad.
You can take your ohm meter and put it across the leads of the
condensor. You'll see the needle/readout deflect toward zero, then
rise toward infinity. On a bad condensor, the needle with not return
as much as it does with a good condensor. A condensor can look good
with the above tests but still be bad, especially at nine volts.
There is a specialized ohm meter called a Megger that uses hundreds of
volts to test resistance. If you put a marginal condensor on this
type of tester, you'll see the needle deflect and not return very
much. It's very evident when you see it on the meter. Considering
how cheap capacitors are, you needn't worry about the specialized
tools. Just get a new capacitor and stick it on the mag.
There are companies that sell capacitors for magnetos for several
dollars each. I prefer to go to the electronics store to get modern,
off-the-shelf components. Tell the guy at the counter that you want a
few 1000 volt, 0.1 microfarad, ceramic disc capacitors. Here they
cost about fourteen cents apiece. Put two of them together, wrap the
leads together, and put it in place of your existing condensor.
Rob
J. K. Siddorn
> Can anyone suggest a Method of checking coils and condensers. I have a
> Villiers Mk20 engine with flywheel magneto, with no spark, and a
> Wolseley WD1 with a weak spark. I assume the problem may be an open
> circuit within the coil, however I am unable to confirm this.
> Any suggestions would be welcome.
>>>>>>>>>>>>>>>
Have you explored each avenue of possible failure? Do excuse me if I am
simplistic ;o))
Points - must be clean and oxide free, flat faced and smooth, set correctly
and open at the right time. They must be properly insulated with appropriate
insulating washers and sleeves.
Capacitor - must have no physical exterior damage and attached in the right
place.
Coils - must also be undamaged and pass current without leaking any to
earth. You can easily check both points and coil for continuity and leaks
with a battery and bulb. You can also check a rotating magnet magneto coil
with a torch battery, opening the points should give you a weak spark at the
end of the plug lead, although I wouldn't expect it to fire a plug. In my
experience, a failed coil is the least likely thing to happen, but it might
be damp. You could try putting the magneto into the airing cupboard for a
few days or the bottom of a warm oven after you've turned it off.
Magnets - Older magnets tend to loose their magnetism and simply banging a
flywheel magneto with a hammer can cause it to loose power. Treat them
gently and NEVER take an armature out of a magneto unless you have to, the
collapsing field and no "keeper" will cause magnetism drop too.
Flywheel - must have a key in the keyway to keep the timing correct.
Plug lead - if it is of the carbonated string type, replace it. If it is
copper cored and rubber insulated, replace it unless there is NO sign of
perishing when you bend it double. Check it for continuity with your battery
and bulb.
Plug cap - take it off and put it to one side until you have tracked down
the problem because they sop up power. If it won't go after you refit it
..................
Plug - the prime suspect! People are so funny about plugs and I've seen
plugs in use that I would not use to prop up a bookcase. I've see otherwise
bright people use ancient old things that should be in a museum, not
attempting to contain 18,000 volts and defeat 600 atmospheres as it attempts
to escape into the fresh air! And others that polish the top of the earth
electrode and put it back in without cleaning the oily soot out of the gap
between the insulator and the body. But my favourite is the pencil trick.
I've seen several different people rub a pencil across the electrodes in
what must be one of the last survivals of sympathetic magic in the modern
world!
At this point, I can do no better than to recommend that you go out and buy
a new plug! NOT clean an old one, but buy and fit (not keep it by you just
in case - seen that too!) a brand new plug of the right grade. If in doubt,
I have a NGK BP6ES in my Wolseley WD1. These engines were designed to work
for a living and they don't, mostly running on tickover on a carb of such
basic and primitive construction as to have been passed by in automotive use
at the beginning of WW One! The manufacturers never expected their products
to be run on tickover for more than a few minutes, so most made little or no
provision for it.
Extended nose plugs have a better chance of staying clean as they carry
their insulated noses out into the combustion space, thus getting any oily
poo swept off them by the flame front. It works, try it ;o))
And that's about it. All you have to do now is figure out which thing - or
combination of things - is at fault. Start with the plug!
Regards,
Kim Siddorn.
The early bird may get the worm -
but the second mouse gets the cheese!
Roland & Celia Craven
took me 10 minutes to revive although some were genuinely dead. Other things
worth checking are: carbon brushes in pickups and elsewhere - are they oil
free, clean and is the spring giving firm contact,
Has corrosion insulated a fixed coil thus destroying the return path -
common where iron/steel pole pieces are fixed to an Alu or Mazak alloy case.
Has over zealous painting insulated the whole mag from the engine thus
breaking the circuit - surprisingly common!
On the Villiers the insulating sleeves can become coated/soaked in a
water/dirt/oil mix that creates a leak path or previous attempts at fixing
have left out one or more of the insulating washers.
good luck
--
Roland Craven
nr Exeter Devon, UK
rc9...@eclipse.co.uk
http://www.eclipse.co.uk/rolands-yard/
"John Lawrie" <john.l...@btinternet.com> wrote in message
news:65b0afb4c1c135b0447...@mygate.mailgate.org...
Nick Highfield
"J. K. Siddorn" <kim.s...@blueyonder.co.uk> wrote in message
news:ah7v64$qvsvm$1...@ID-151486.news.dfncis.de...
> But my favourite is the pencil trick.
> I've seen several different people rub a pencil across the electrodes in
> what must be one of the last survivals of sympathetic magic in the modern
> world!
>
My father did his national service as an airforce ground mechanic and
recalls that the Stuart-Turner engines (P5?), which drove the pump on a fuel
bowser, were often given this treatment to encourage starting on a cold damp
morning. Apparently it worked, though nobody could ever say why- perhaps you
are right and it was some kind of offering to the deities of infernal
combustion!
I once wrote this up for SE magazine, suggesting that others should write in
with their favourite 'dodgy dodges'. Unfortunately the editor and I were not
on particulary good terms at the time (all ok now!), as I had taken
exception to the mangling of one of my articles, and the letter was never
published. 'Still, if SE won't takle this vital subject, I reckon it would
make an interesting thread on this group. Any takers?
Nick H.
Philip Thornton-Evison
"J. K. Siddorn" wrote:
> But my favourite is the pencil trick.
> I've seen several different people rub a pencil across the electrodes in
> what must be one of the last survivals of sympathetic magic in the modern
> world!
>
Well, call it sympathetic magic or whatever else you like, but I can assure you
that it DOES work!
Especially if the mag. is a bit under par it can often make the difference
between an engine that will start and an engine that won't.
Softer pencils are best, presumably because they have more graphite in them.
Somewhere at the bottom of my toolbox lurks a stub of 4B pencil kept especially
for starting purposes. Not on my own engines I hasten to add :-)
Remember that no matter how good the mechanical condition of an engine, if the
mag, is dodgy, it ain't gonna run.
Conversley, a good hot spark can coax the most knackered and disreputable
engine into wheezing, smoking life.
Regards
Philip T-E