Testing a Magneto.

[thrang]

Any one know how I can bench test a Magneto? I replaced the points and plugs on my A7's and I'm still getting a very weak and crappy spark. So looking for any advice on testing the thing.

Thanks
Tony

[Tony Smith]

Any one know how I can bench test a Magneto? I replaced the points and plugs on my A7's and I'm still getting a very weak and crappy spark. So looking for any advice on testing the thing.

Back in the day you tested a magneto by "feel", they were true "hard men" do not try that today.....


Seriously though, Aside from the points the problems that afflict magnetos (and I am assuming Lucas variety here seeing as they are on an A7) are in more or less approximate order:-

Breakdown of insulation in the plug cap, plug wire or pickup.
Loss of magnetism - to determine this you need a good one to compare against, or a lifetime's worth of experience in fixing the misbegotten things.

Failure or Partial failure of the insulation on the coils -you will not have the equipment to test more than static resistance - do you know what the value should be?

Movement of the "flux point" - a mystical concept I have never quite fully understood, but I have taken a weak magneto that to my testing had perfectly good insulation, serviceable winding, new points but really weak spark and the old guy has said something like "the flux point has shifted, leave it with me and pick it up tomorrow" .the next day, along with a bill for $10 or $15 I would walk out with a perfect functioning "re-fluxed" magneto. Now I may have just been sold a quart of halogen fluid to top up my parking lights, but I didn't care then and I don't care now. I learnt all I reckon I needed to know about magnetos a long time ago, and it distills to the this.



1/. There are two sorts of magnetos int he world. The sensible German ones where the easily balanced and hard to damage magnet bits rotate inside the fragile, impossible to balance coils, and the bizarre English ones where the coils rotates inside the magnet.

2/. German magnetos have long trouble free lives and English ones don't.

3/. there is a further derivation of English magnetos made by Joseph Lucas, they double don't!


4/. You can yell and swear and fiddle with a magneto that isn't working properly as long as you like and throw as many brand new parts as you can afford at it and it still will not work. Or you can take it to one of the grey-haired bearded experts who operate from tiny shops in the back streets and they will lay on hands, make the correct incantations and the magneto will work like a Waterbury watch.


Serious mode back on.... Yiou need a device called a "Megger" to properly test for insulation partial failure, and even if you are able to prove that the coil is faulty, do you have a coil winding jig, or the time to make one and then the time to use it? Rewinding coils is not rocket science, I rewound th elighting coil on my Yamaha years and years ago because I didn't like the price of even a 2nd hand one from the wrecker. BUT somewhere after the 1st 6-pack I obviously lost count and my lighting circuit puts out 19v and destroys regulators and batteries more or less on a yearly basis - I've got kind of used to just putting a new battery into ti every year and having a second regulator already bolted to the frame, requiring only the plug to be switched over if the lights got out or the battery goes flat.


So, after having bored the hell out of you to this point - take it to an expert and have it fixed expertly. Your sanity will thank you.


Oh, and the best solution to a Lucas magneto is to replace it with points coil ignition - you can use the magneto points and one of those lovely miniature double ended coils by PVL that are easily hidden.

Boyer also make kits for English Bikes.

[Motu]

A mag puts out a pretty weak spark just kicking over, and gets better the faster it turns, one reason those things were hard to start.  Check your pickups ( I pressume black and clip in, if you have red screw ins it's a comp mag), also pressuming it's a Lucas mag, BTH  were a far better product.  Also check the slip ring, they are prone to wear, it needs to be smooth allround, not a step to the brass bit.  No resistor caps and no resistor plugs, you need all spark you can get.  Auto or manual advance?

The thing I don't like about Lucas mags is the discrepency between cyl timing...as bad as 10 degrees at times.  Caused by the housings being out of line.  You could fart around to try and fix it, but I used to toss the Lucas and fit a BTH, or distributor.   Anyway, when you set the timing, check both sides...they won't be the same.  That's why later British bikes had dual points, so you could get both sides the same.

[skippyc]

Magnetos have condensers as i don't know the actual one you are talking about, it could be the lump of metal strips on the end of the rotor or mounted somewhere else. The mounted somewhere else ones can fail overtime the old ones don't seem to fail much at all.

The magnet flux thing works best when the points open just as the rotor and pole part company. A manual advance shifts this point and makes it difficult to get a spark when advanced with the magy turning slowly.

The stronger the magnet the better the spark, it not uncommon to need to re-energize magnets.

And last but not least the later lucas magnetos used on stationary engine are still the best ones around. they must have learnt something after all.

[thrang]

A mate of mine who's into his vintage cars came and took a look at the mag and declared it 'shagged out' so its off to the auto ellecy in the morning for fettling. After years of working on Beemers working on the SS is a whole new learning curve, but bags of fun. Even though I'm going to spend a lot of time un-bodging the half baked repairs done by which ever 'genius' read complete Muppet, worked on her in the past. That said I did buy her knowing she was priced cheaply because it was a cosmetically restored, but non/rough running example of the breed.

[Tony Smith]

And last but not least the later lucas magnetos used on stationary engine are still the best ones around. they must have learnt something after all.

don't care - still hate the breed. That aeroplanes are required to be fitted with two of them is a statement of faith in them by the entire aviation fraternity.

[skippyc]

During the early seventies a mate did his national service in New Guinea and they had caribou aircraft. They would go through a lot of magnetos.

[thrang]

Got the Mag back looking all new and spiffing. The total cost for the work by a local auto and classic boat electrician was £80, which about half the price the classic bike specialists were quoting just to look at it!

[wilcom]

caribou aircraft

I have a soft spot for that plane. When I was in the Army I took numerous flights. That thing could land on a postage stamp!!!!

[skippyc]

I read a story of a pilot in Vietnam flying one into a small airfield with a strong head wind and the slope of the field was down hill. He couldn't get it to drop down onto the strip so in desperation he threw it into reverse thrust which dropped it onto the strip.
Evidently they are the only plane that will go into reverse while still in the air.

[Tony Smith]

I read a story of a pilot in Vietnam flying one into a small airfield with a strong head wind and the slope of the field was down hill. He couldn't get it to drop down onto the strip so in desperation he threw it into reverse thrust which dropped it onto the strip.
Evidently they are the only plane that will go into reverse while still in the air.

Pilatus Porter can too.

A number of jets used to be able to, then Lauda air lost one due to an un-commanded, asymmetric thrust reverser deployment and now they are universally locked out and only able to engage when weight is on the wheels.