Magneto Problem

Magnetoman

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(note to moderators: if you feel it would be better if I ignored future posts to this thread, just tell me.)

Max, I thought about this for an hour before deciding I need to respond because the incorrect information in your post only would make it more difficult for Bruce to fix his magneto and would continue to misdirect anyone else who reads it. But, I don't want to start an argument, just correct your misunderstandings about the operation of magnetos.
Get a couple of the hottest heat range plugs with the smallest center electrode, these plugs are of a low resistance. This is the best way for the garage mechanic to go if a weak magneto is in question.
The heat range has nothing whatever to do with whether or not a plug will spark. The resistance of the plugs is completely independent of heat range; they all would be the same. Although the type with small center electrodes will spark at a slightly lower voltage than the others, because the electric field is somewhat larger near its tip, that is not the problem Bruce is having. And it only would mislead a mechanic trying to diagnose problems with his magneto because swapping plugs with different heat ranges would tell him nothing under these circumstances.
Your Vincent will at idle with a good mag jump a half inch gap with ease, ... A Vincent in good nick will be able to jump a gap of 2 inches in the atmosphere running on one cylinder at around 3500 rpm.
This is both false and damaging. The internal safety gap is only ~0.35" so the spark will happen internal to the magneto rather than across an external 1/2", let alone a 2"(!), gap. Also, the safety gap screws are there to limit the voltage to ~25 kV to minimize the chance of permanent damage to the coil due to arcing between the windings if the voltage were allowed to go any higher. No matter what, and especially with an old coil, pushing it to its limit by relying on the safety gap screws to drain the voltage is a very bad idea.
Again, put it in a dark room and see what is going on.
Someone is likely to see corona discharge in a dark room, but under most circumstances corona discharge only drains off an inconsequential amount of energy from the spark. Although Bruce might see a glow in his dark garage, the likelihood is that following your advice would cause him to unnecessarily waste his time trying to eliminate it when the actual problem is elsewhere.
 

Kansas Bad Man

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The heat range was not a reference to a better spark, it was only if the engine was oiling the plug causing a possible problem. I am fully aware of the spark plug heat ranges and there effect in real time operation. As for the small electrode, one only has to go to most spark plug charts that recommend the spark plug to use under what circumstances. You will find that a small electrode is recommend for magnetos with a week spark. All that I have recommended is good advice , will not hurt your magneto in anyway, and the tests I recommended will not take but a few minutes and not a waste of time .
I have not meant any of my words to correct a malfunction in its totality of the magneto on one of its spark leads. I have fixed a number of Vincent's with similar problems , I speak with some what authority in this area. Owning and operating 4 motorcycle shops this if I might say this is rather elementary. A Vincent that is not firing on both pots that cant be fixed in not more then 1 to 3 hours better find a different business to get into . This is my last post on this thread.
 

Ken Targett

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Going back to Bruce's original question, if you'd like a step-by step guide, Bruce, to just about everything that you can check on a rotating-coil magneto, most of it while it is still on the engine, without the need for anything more exotic than a multimeter, please take a look here:
(I've been meaning for quite a while to add some photos to it, but at the moment it's just words.)
Ken.
 

bmetcalf

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Thanks, Ken, I'll go through the steps.

I have been doing some parts swapping. I get no spark at the rear plug when I have its wire connected to the front brush holder, but I do get a spark when it is connected to the rear brush holder. I have ordered a new holder from VOCSC, so we'll see if that is it.
 

Kansas Bad Man

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After reading Kens link, it is apparent that I have misinformed the readers of my post. Both Ken and the other guy who are the magneto experts say that to spark the magneto HT LEAD to ground may and does cause damage to the magneto. I have repaired many magnetos over the years and my procedures are in almost all respect the same as the very well written service procedures by Ken. Having sad that, my saying about atmosphere spark testing is a fact. Always, after I have performed a magneto overhaul , I put the mag in a vice and spine the magneto with a 3/4 inch variable speed drill in direction of magneto rotation. Most drill motors are of the 3000 RPM variety which would test the magneto at a engine RPM of 6000. My test is with sparkplugs at first, both plugs grounded to magneto housing. the next test is with the HT leads removed a set of Honda sparkplug leads with low resistor plug ends a test is made to see if a adequate spark to ground can be obtained, this will insure that the plug will fire in real life. This is a short duration test, and I have yet to hurt any magneto in any way over the years using that test procedures. Call me lucky I guess. I would only add to Kens overview is a paragraph on the checking and repair of a 50 degree cam ring of a Vincent that is say 46 or 47 degrees.
 

SteveW

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Bruce,
If it is not the brush holder, note that the front cyl.(#2) inherently fails first before a complete failure due to the degrees not matching between 50 degs (cyl heads) and 180 degs. magneto flux. The rear cylinder was given precedent over the front for some reason. I'd suggest, the mag needs a rebuild w/ a new capacitor. It is probably hard to start hot.? Good luck.
 
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