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Magneto polarity

nkt267

Well Known and Active Forum Website User
VOC Member
I have been wondering about the polarity of the spark from a K!F.
The cam ring and the point carrier positions are fixed by the design of the mag to give the spark where maximum flux occurs,give or take a bit.
Does it make a lot of difference to the spark/running of the engine if the cam ring housing is turned by 180 deg.
If it does how can you tell which way up the cam housing should be..john
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
I have been wondering about the polarity of the spark from a K!F.
Does it make a lot of difference to the spark/running of the engine if the cam ring housing is turned by 180 deg.
Yes, it does make a difference, but whether or not it is "a lot" is a value judgment. Because for a given voltage drop the local electric field (Volts/cm) is higher near a point than near a flat surface, it is easier to emit electrons from the central wire of a spark plug than from the earth electrode. So, the polarity does matter.

On a twin you have no choice since the plug on one side of the engine always will be at the "wrong" polarity. It's even worse with a Vincent twin since the magneto provides maximum output with the armature exactly 180-deg. apart, so not only will one plug always be at the wrong polarity, only one plug can be at the optimum point of the collapse of the magnetic field. That's why with a Vincent twin you at least want the plug on the non-optimum side of the magneto to fire when the central electrode is negative so you don't have two factors working against you.

While a single certainly will work with the "wrong" polarity (just as both cylinders on a twin do work), you will be better off it fires when the central electrode of the plug is negative, not positive.
If it does how can you tell which way up the cam housing should be.
As originally manufactured the cam had an indexing slot to make sure you couldn't put it in the wrong way. But, 60 years later, there's no telling what parts are in a given magneto. I've seen illustrations in books showing how to hold a sharp pencil tip between the electrodes to determine which way the electrons are flowing, but how one actually holds a plug and a pencil in a dark room while kicking over a bike and observing the tiny spark bounce off a pencil is a problem I've never had to solve, because I have electronic instruments to spare me from that pain and frustration.
 

greg brillus

Well Known and Active Forum Website User
VOC Member
The difference in output voltage is directly proportional to the primary voltage of the coil. Correct..? Ok, when on the test rig and running at say 1200 to 1500 rpm which is half crank speed. If you check the primary voltage output with a PEAK VOLTAGE METER with an analog needle, not a digital one.....the voltage shown on the gauge will be around 200 volts.....the ratio of primary windings verses secondary windings is 60:1 so 200 volts x 60= 12000 volts......correct.....if you rotate the camring housing 180 degrees, the primary coil output at the same test RPM will be 120 volts.......Believe it or not..? It is absolutely true.....so there is your answer.....it definitely would have to make a difference. Magnetoman would know more than I about this than I, but I have tried this many times on single cylinder type magnetos. And it seems to happen on all of them. The man I learnt all this from was one of the best magneto repairman in Australia......Luckily I spent 7 months learning from him before he passed away 3 years ago...for those in Australia that may have known him, his name was Ivan Brown.....an auto electrician by trade, a total motorcycle nut, and a very clever man indeed.......Cheers.....Greg.
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
There's this tool also: ...It might work with a magneto.
It almost certainly would work with a magneto. However, a voltmeter would be enough. Some voltmeters have switches to reverse the polarity, and many allow the red lead to be plugged into the black socket (and vice versa). But, if you keep track of the polarities, set the voltmeter to DC and on a high setting (500V, or whatever), you could connect one lead to the housing and the other to the spark plug lead and do a test with only small risk to the voltmeter. If the magneto is off the bike, turn the armature by hand and let it flick past the position of maximum physical resistance on its own (i.e. at the lowest possible speed). If the voltmeter registers a negative pulse the polarity is correct. If it's on the bike do the same thing but with the plug removed from the engine to make it easier to turn the engine over slowly and controllably.
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
p.s. for work like this an analog voltmeter is best. A given digital voltmeter might work, but they update their displays periodically (vs. continuously with analog), and that update frequency can, and often does, interact with the changing input voltage in ways that make it confusing to know what's going on. When I'm doing any kind of measurement like this I always reach for a trusty old analog meter rather than any of my "far better" digital instruments.
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Hi Magnetoman, I read something in the last couple of years that I think you had some input to, concerning remote fitting of condensers/capacitors for magnetos being impractical? Is that because of the increased Inductance/inductive reactance of the conductors or something else altogether?
Regards
Kevin
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
... remote fitting of condensers/capacitors for magnetos being impractical? Is that because of the increased Inductance/inductive reactance of the conductors or something else altogether?
It's something else altogether. The simple answer is the capacitors being sold for this are not rated to survive the high pulsed currents they will experience in a magneto. I describe this in a sidebar on magneto condensers in the following post:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=455839#Post455839

Unlike the condensers on older cars that were made as cheaply as possible because they were easy to access and would be replaced as a regular maintenance item along with worn points, the internal condensers for rotating armature magnetos were made to last "forever" in a magneto. In the case of pre-WWII mica condensers forever what pretty much forever, but for post-WWII paper condensers forever was a few decades. The reasons these post-WWII condensers slowly, but inevitably, die are described in:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=466579#Post466579

However, I conducted accelerated stress tests on appropriate modern replacement capacitors that are rated for high pulsed currents and identified two that will last "forever." These tests and the capacitors are described at:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=467734#Post467734
and
http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=508762#Post508762

For the first capacitor "...my conservative estimate is there is a very high probability these Panasonic capacitors will function without failure in a magneto for at least 140,000 miles or 40 years." For the second one, "Although these tests were not as extensive as the ones I conducted on the Panasonics... Taken in combination with the manufacturer's specifications, and the fact they have the same internal polypropylene structure as the Panasonics, I would have no hesitation using them myself if I did not already have a very large stock of the Panasonics."
 

Ken Targett

Active Forum Website User
Non-VOC Member
Going back to John's original questions, as Magnetoman suggests, the relative shapes of the sparking plug electrodes affect the ease with which a spark can be produced. Also, as mentioned in the Chicagoland MG Club article, the relative temperatures of the sparking plug electrodes also affect the ease of spark production. I'd always been led to believe that with traditional sparking plugs, it's the temperature that has the greater influence. Whether that's true or not, I don't know, but it's of no matter because both shape and temperature indicate that, with a traditional sparking plug, the non-earthed centre electrode is preferably negative. Rotating the end housing of a K1F 180 degrees will indeed swap the polarity of the magneto output.

As to John's second question, the end housing should, I suppose, be the way up that Lucas put it originally, and I think I'm correct in saying that they always fitted them with the channel for the cam lubricating felt at the bottom. Whether that results today in the preferred polarity of the HT output then depends, of course, on the history of the magneto. For example, has it had its magnet's polarity swapped? Has its armature been rewound incorrectly? At least with a K1F magneto, the brass end piece of the armature at the contact breaker end only fits the bobbin one way. With many other magnetos, the end piece can be fitted upside down and swap the HT polarity as a result.

To test the polarity of the HT output, the pencil flare method does work, but does need an experienced eye, and as Magnetoman suggests, you'll probably need a helper if the magneto is on the bike. With the meter method, don't use a digital multimeter. (I'd have thought there's a distinct risk of blowing it up, and they're not at all good at telling you anything about brief pulses.) A simple moving-coil ammeter (anything between perhaps 100 microamps and 10 milliamps) with its negative terminal connected to the HT output and positive terminal connected to the magneto earth will twitch up the scale when the points are opened slowly if the HT output is negative, or swing back against the end stop if it's positive. The Snyder polatity (sic) tester should work. If you want to make your own, simply connect the anode of a red LED and the cathode of a green LED to the HT output, and the cathode of the red LED and anode of the green LED to the magneto earth. If the red LED flashes when the points open, you've got a positive HT output, or if green flashes, you've got negative. (When doing this, it's important that both LEDs remain connected together, otherwise you'll likely blow one of them.) Another method is to use a 3-point gap as described on pages 366 and 367 of http://naca.central.cranfield.ac.uk/reports/1925/naca-report-202.pdf. If the spark tends to bend away from the third teaser electrode, you have a negative HT output. If towards it, then positive. I've even heard it said that if you stuff your finger in the HT output and turn the magneto, you can tell the HT polarity from the shocks you get. I get enough shocks accidentally from magnetos without wanting to try this to see whether it works. Obviously, not to be attempted if you have a weak heart or pacemaker.

If you work out that you've got the wrong polarity with the end housing the right way up, you can either fit the end housing upside down, or if you have access to the required equipment, you can swap the polarity of the magnet.

Ken.
 
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