Misc: Ignition Series 'C' Magneto

[lotus15racer]

I'm in need of general advice on the Lucas KVF.

The magneto is assembled, not installed. It needs to be completely serviced. What do I look out for? Can any recommend a shop to rewind etc.

The magneto has not run since 1997, and may never have been rewound or serviced. Bike is on coil ignition now, want to reinstall the mag. Many thanks for advice.

Steven

 

[chankly bore]

The authoritative text on magneto overhaul can be found on the Britbike Forum under the heading "Restoring a rotating armature magneto". the author is Magnetoman. I believe there is a link on this Forum. Rewind, new modern capacitor and re-magnetising are mandatory. Sit down with a cup of coffee or a bottle of sterner stuff and read it all, it is worth the effort.

 

[Chris Launders]

Every thing other than the rewind and the remagnetising you can easily do yourself, you will need a bearing puller, they are about $15 on ebay or make your own, all the bits, bearings, slip rings, paper washers and shims are readily available. Taking it apart make sure you take out the spark earthing spikes first. For a good rebuilder go on a pre-unit Triumph/ BSA or Norton forum as there are many many more of them than Vincents

 

[Peter Holmes]

The problem with trying to do the work yourself is that once you have pulled it apart does it not need remagnetising anyway, not a job you can easily do without the necessary equipment, where is Magnetoman when you need him?

 

[bmetcalf]

See if the NorCal or SoCal section organizers have a preferred repair shop. With your fine weather, you guys ride a lot and need mag work more often.

 

[Magnetoman]

You'll find everything you need to know about rebuilding magnetos, much that you didn't know you needed to know, and a lot you won't care to know, at:

Restoring a Rotating Armature Magneto

Restoring a Rotating Armature Magneto Table of Contents (Note: I added this Table of Contents after uploading the final installment of this thread. Clicking on an underlined link will open a new window with the individual post) Table of Contents, Background, Introduction Initial Insp...

www.britbike.com

Even though you don't intend to rebuild it yourself, the information will help you ask the right questions to determine whether or not you should trust your magneto to a given rebuilder. For example, if a potential rebuilder doesn't use capacitors like the ones recommended in that thread, doesn't have an appropriate magnetizer, automatically suggests a "rewind," etc., you will learn from that thread why you have to find a different rebuilder.

 

[MarBl]

everything you need to know about rebuilding magnetos

I wonder, if it possible to replace the magnets with modern SmCo.
Those are now even offered with individually defined shapes. Appropriate modern glue is also available, so I would guess that it all comes down to the question, whether the old magnets can be removed without damaging the housing.

 

[Magnetoman]

I wonder, if it possible to replace the magnets with modern SmCo. ... whether the old magnets can be removed without damaging the housing.

Although just about anything would be possible, basically the answer is 'no'. Also, this is a case where 'more' wouldn't necessarily be 'better'.

SmFeCo magnets have greater field strength so would induce greater current in the armature than with the internal Alnico magnet. But, without experiments to test the result, it's not clear the fine wire in the armature would survive the higher current. Even if it did, and even with a proper condenser, the points would erode faster. Further, a Lucas manual says the 300 rpm is the lower limit of kickstarting speed, and a properly remagnetized magneto will provide sufficient spark even lower than that. If your knee can't handle supplying 300 rpm, would you be willing to see if it survived the, say, 250 rpm a higher strength magnet might provide?

If I were to design a magneto from scratch I would do so using a SmFeCo magnet. But, I wouldn't retrofit a Lucas KVF with one. It would be easy to attach SmFeCo magnets to the outside of a KVF body, but it would be a mistake to do so for the reasons given in the previous paragraph. As a note for anyone tempted to try, a SmFeCo magnet has enough strength to significantly demagnetize a KVF when attached to the "wrong" magnetic pole, but it doesn't have enough strength to sufficiently remagnetize it afterwards when attached to the "right" one. You've been warned.

p.s. somewhere in one of the Appendices to the magneto thread on Britbike I have photographs of a magneto housing that I cut open, which lets you see where the Alnico magnet is located.

 

[MarBl]

not clear the fine wire in the armature would survive the higher current

The idea would be, not to get a better spark, but achieving the same performance with a thicker and therefore more robust HV wiring.
But I guess there are so many unknowns that this would be a rabbit hole to get lost in very quickly.
I really liked the magnetos but the uncertainties within those rewound armatures made me go electronic. If there would be a straight path leading to a reliable magneto, which to me is equivalent with a reliable HV wiring, I would be back in no time. (I even stocked some of the capacitors you recommended in your excellent thread)

 

[Magnetoman]

If there would be a straight path leading to a reliable magneto, which to me is equivalent with a reliable HV wiring, ...

My observation is that most of the "unreliable" reputation magnetos have with some people is a direct result of magneto repairers who didn't know what they're doing. As detailed in one of the Appendices of my restoration thread, the type of less-expensive condenser developed at the time of WWII, as a result of lack of access to a supply of Indian mica, has a limited lifetime due to the internal chemistry of the components.

When the original condensers inevitably started failing, people who didn't understand there is more to a capacitor than capacitance started installing ones lacking the capability to handle the large pulsed currents generated in a magneto. Articles were written in various places, including 'MPH', recommending capacitors that were doomed to fail. Other articles in 'MPH', including one by an electrical engineer who shall remain nameless, made other faulty recommendations. Further, even if a correct replacement capacitor is used, the inevitable reduction in magnetism when an armature is removed results in reduced performance of the magneto. Instead of having enough spark to start an engine at 300 rpm, it requires older men with weaker knees to kick harder than when they were 18. Some magneto restorers still insist an "internal keeper" design keeps the magnetization from dropping when the armature is removed. This only would be the case if you believe in magic, not physics.

In most cases, I assume that someone who recommends "rewinding" a magneto doesn't understand magnetos. Failure of the windings does happen, but it's fairly uncommon, can easily be tested with appropriate equipment, so isn't a prime suspect when there are problems. Some years ago I "inherited" a large quantity of disassembled magnetos. I have no way of knowing what failure caused each of them to have been disassembled, but most of the coils are fine. If I were rebuilding a magneto with a bad coil, I would use one of those old Lucas armatures (after stress-testing it) rather than an armature rewound by some unknown person who may or may not know what they are doing.

A properly rebuilt Lucas magneto will be very reliable for decades, requiring a minimum of infrequent maintenance (lube the points cam, clean the earth ring, replace the brushes, etc.). More reliable than a battery-dependent ignition system, since those depend on a battery that can and will die after a few years, and when it's most inconvenient. A properly rebuilt magneto is more reliable than any electronic ignition system as well, since in the end all of them depend on an undependable battery.

Leave a magneto-equipped bike sit for a year, add fresh gasoline, tickle, choke, retard the spark, and reasonably expect it to start on the first kick. Try that with a bike that relies on a battery for the spark, even if it has been connected to a charger the entire time. In the latter case you hope it will start, while in the former case you can reasonably expect it to.