Jap 2A Magneto

What resistance reading are you getting from the coil?

About 6k ohm

That looks high for a secondary, but look at the primary coil resistance as well, that should much lower, probably 25-50 ohms.

Secondaries do vary, but I'd have guessed that is possibly open circuit with carbon across the gap/break,

Roly probably has some bits he can measure up, I'm at work so I have not got anything here to lay a meter on, other than an old Suffolk Punch.....

Peter

Hi Paul, One way of testing is shown at

than that there is nearly always someone with a meter & the know-how at most rallies. Where are you based?

The primary resistance is about 0.75ohm

I am based in South London.

Paul

6K is not inconsistent with past results I've had from good mags of this type but there is no guarantee its not breaking down under load. Our resident capacitor man is getting wet at the moment but will hopefully return soon. Bitter experience suggests checking all the obvious things first :-) Is the HT lead sound and with sound connections at both ends, are the points absolutely clean with no trace of oil or oxide film, and the one I always fall for; are the insulating washers in the right place? From my one experience of this type of mag the coil/pole piece distance from the flywheel is adjustable and needs to be not quite touching. hth Roland

"....are the insulating washers in the right place?" I don't know for certain. I assumed that the engine had just been put away under a bench for the last 25 years, but if someone played with the ignition and put it together wrong, that could be why it was put away.

I am getting a connection to the casing from both the terminal and the body of the capacitor - but when it is removed, there is no connection between them.

There is an lv wire that goes through a grommet into the crankcase - I can't think what it is for other than earth, but if it is why not terminate where you can get to it ?

Does anyone have a wiring diagram ?

"...the coil/pole piece distance from the flywheel is adjustable..." It does not look adjustable.

Paul

Hi Paul, testing the capacitor fully (often the archaic, but contemporary, term condenser is used when talking about magnetos) is not really possible without equipment to subject it to 500V DC and measure the extremely small (if its any good) leakage current when fully charged. A measure of the capacitance and power factor also give indications as to the quality of the capacitor. 500VDC in the primary circuit may seem a lot to some of you, but I have it from two good authorities that a goodly back emf will not be kind to capacitors of lesser voltage rating.

I have to say that you would need to look far and wide to find someone who knows less about JAPs than me, but magneto principles are pretty similar in most engines. The capacitor is used to suppress sparking across the contact breaker points in order to stop the points becoming pitted and useless, and to allow optimum energy tranmission to the sparkplug. Typically in the latter stage of a capacitor's life it becomes leaky, eventually going short circuit at working voltage. This means a lot or most of the energy meant for the spark is lost.

I suspect that the capacitor in a JAP is oil impregnated paper/aluminium foil. Perhaps some of the others could comment on this point. If its a mica capacitor, they can last for much, much longer. In my experience, any oil impregnated paper capacitors of this sort over thirty years old which I've tested are knackered. I've tested all the condensers from a job lot of about six Wico A mags I picked up once, and every capacitor was almost short circuit at 500V.

As to capacitance value of the magneto capacitors I've measured are around

220nF. I must stress none of these are JAP capacitors. For any of you who are wondering, duff capacitors usually measure okay on a capacitance bridge which subjects them to a couple of volts AC.

Paul, sorry this is so wordy, its written to be informative in general. The measurements you took on the coil sound quite reasonable to me.

Now then, Roland, as to your remarks about getting wet, the only problem I suffered from weather wise was sunburn! I'm still aglow now :-).

Regards, Arthur G

It obviously does not rain on the Godly:-) We had sunshine and showers on Sunday and it persisted down yesterday. regards Roland

suffered from weather wise was sunburn! I'm

Thanks for that, but without any capacitance metering facilities it seems that my best way of eliminating the capacitor is by substituting a different one.

How do I find out what specification to get - there is nothing marked on the Wico one, nor is there anything marked on the capacitor on my rather rusty pile of Matchless (Which may or may not work).

Can I damage the coil if I get it wrong ?

Paul

condenser is used when talking about magnetos) is

the extremely small (if its any good) leakage

also give indications as to the quality of

you, but I have it from two good authorities

knows less about JAPs than me, but magneto

suppress sparking across the contact breaker

allow optimum energy tranmission to the

leaky, eventually going short circuit at

foil. Perhaps some of the others could comment

longer. In my experience, any oil impregnated

knackered. I've tested all the condensers

capacitor was almost short circuit at 500V.

220nF. I must stress none of these are JAP

measure okay on a capacitance bridge which

The measurements you took on the coil sound

suffered from weather wise was sunburn! I'm

Paul,

I would try any condenser from a motor factors or similar, the capacitance value is not all that critical and I can't see how you could damage the coil.

Regards, Arthur G