Testing a Mag at home ?

[john.nash]

I got a magneto, with my latest project.
It sparks when run, via a drill, in my vice.

How would I check it further ?
I am thinking a moving coil meter on the output and then run it for a bit and observe any decrease in output ?
Or should I just assume it's shagged and send it down for rebuild (not cheap)

How hard is it to change the condenser yourself ?

cheers

John

[nevhunter]

John, some postwar coils seem to last well enough, but unless an earlier one has been rewound, it's not likely to do more than one good run before the " shellac" runs out of it and locks the armature in the case.
If it has a good low speed spark the condenser must be working. I often keep the mica condensers. if they are working. Some people just replace them with "modern" ones They are not real easy to replace as you have to locate them in the end of the armature where the original was a good fit.
If the coil is soft to the touch it has had it.
You normally test for faults by running ( motoring ) it for perhaps two hours. With about a 5/16 inch gap at say 1200 rpm I sent a magneto back to a rewinder some years age because I thought it was breaking down. He had to run it for over 2 hours before the fault was replicated. He was going to pass it as OK and instead went and had a cup of coffee and it eventually sparked intermittently and failed. Older coils have absorbed moisture and the Lacquer? oxidises and some of the current leaks and they get hot and fail. I have NEVER got away with running an old coil for more that a couple of days outtings. Nev

[keith.mettam]

I agree with Nev. If the armature is original you are wasting your time using it for the very reasons he has quoted. Rebuild the mag yourself John. Get an exchange armature, new slip ring, new bearings and insulating washers, new ht, new pick up and brush, new earth brush. Refurbish the existing points yourself as they are usually in fairly good nick, if not then replace them as well. You can probably rebuild the thing for under half the cost of getting an expert to rebuild it and also have the satisfaction of having done it yourself and knowing you have done a good job John. Watch out for armature end play or binding when you assemble it and shim accordingly.
All the best,
Keith Mettam.

[brenton.roy]

Hi John,
I'm no expert with magneto's - all the above sounds good advice. I understand that if you pull it apart, you should put iron strips over the magnets or they will lose their magnetism?
Does anyone know about this - or am I on the wrong track?
Brenton

[pete.collings]

I believe this was a potential problem with earlier magnetos, although later ones are supposed to be manufactured with High Hysterisis material that will retain magnetism with minimal loss. Heat is more often the cause of loss of magnetism, as anyone with an overheated magneto can verify, as they wait for it to cool by the roadside. A slipping drive clutch on a MOI type magneto is a sure way to overheat it, I always remove all trace of grease on the clutch surfaces, only putting grease on the driving teeth once everything has been assembled.

Having said the above, there is nothing to loose from placing a "keeper" in the mag body when dismantled, a girling shock absorber cover is a good size if you don't have a spare armature. Remagnetising needs specialised equipment, I believe a high current is passed through a copper coil for a short time, with the coil placed inside the magneto body.

[john.nash]

How can I tell the age of the armature and whether it's been rewound ?

I have the same decision, with the wheels.
Do I spend hours and hours (that I don't have; what with work the way it is at the moment) learning to rebuild a wheel (two here waiting to be done) and then perhaps never do another for 10 years ???

The more I read, above, the more I am thinking "send it off" ...
On the other hand, if someone around here knows how then I could fix his computer and he could show me how to rebuild a mag ?

[brenton.roy]

Hi John,
If you used Keith's theory - buy the new bits and put it together, it's unlikely you will get into too much strife. I don't think they are too complicated until you start trying to rewind things yourself.
At worst, you have a good crack at it, give up, pack the bits in a bag and pay someone to assemble them.

BTW, wheels are easy. In the same way as above, they are good fun to rebuild, so long as there isn't a major problem (wrong length spokes, bent rim etc).
Just use an existing wheel as a pattern.
Brenton

[nevhunter]

If the coil "looks" fresh and is hard when you press on,,.maybe it has been rewound at some stage.
I would just send the armature away and do the rest yourself. I remagnetise virtually all the magnetos I fix. I have a professional magnetiser which It would take two people to lift.If magnetos have been left with the armature removed and rattle around in a box, there is not much magnetism left after a while. Sometimes someone has tried to remagnetise it and done it backwards. That doesn't help things.
John if you are going to remove the bearings you DO need a proper puller as they are often quite firm on the shaft. Do not hit the armature at all as they are quite flimsy and the brass parts distort and the whole thing runs out of true. Some repairers skim the armature to make it look nice but they increase the air gap and the magneto performance is reduced. Magnetos don't give much trouble once they are done right, they will run for years. Nev

[john.whiting]

A simple way to test a mag on a running bike is to ride the bike for an hour or so then shut it off .If its hard to start,has to be pushed,or run down a hill,the the mag needs a rewind.Lucas mags are hard to remagnetize,because you cant directly apply the poles of the magnetizer,like a BTH or an early mag with horseshoe magnets.I think everyone knows that radio interference suppressed components are fatal to mags.This is not only wire,but plug caps too.Incidently, in most countries it is illegal to operate a HT ignition system without suppressed components.Catch 22.Regards John.

[simon.holyfield]

I'm with Brenton on the wheels. My son pulled a '60's Raleigh out of a skip, and rebuilt both wheels. He was about 14 at the time.

A couple of years ago I rebuilt both my Bantam wheels, using the jig idea in the Radco book.

Brenton's right - it's fun and it's easy. Don't use plated steel nipples though, they rust!