Misc: Ignition Lucas KVF Rebuild - UK Recommendations?

[Chris Launders]

Chris. I did see a magazine article a good few years back where the writer explained how he had ground a face cam to work on a V twin - sadly, I didn't file away a copy! Doh! It does show that it can be done but Lucas never made any face cam twin (V or otherwise) magnetos as far as I know. The body would have to be machined to accept a second pickup of course. It would also be necessary to change the slip ring as the single cylinder ring has a continuous brass band for the pickup brush. The twins would need a slip ring with a short brass segment which passes under one pickup brush at a time. You would also have to accept the best spark/retarded spark situation of course.
The MO1 magdynos were all face cam single cylinder magnetos. Similar looking twin cylinder magdynos of the same era (separate dynamo clamped on top rather than built into a common body as done on earlier magdynos)) were available for parallel and V twins. They were designated MS something or MN something depending on the variations. They all had ring cam points assemblies.

Yes when building the MO1/MSL hybrids I needed to fit the appropriate rotation slip ring and machine the 2nd pickup hole.

There is an article with I think all the information of modifying a face cam in a Brough Club publication I have, I will dig it out tonight and review it, if it's of any use we will have to work out a way of getting a copy to you (the forum won't allow me to send you a private message, I've tried already)

 

[timetraveller]

As a last resort you can send it to me and I can print it out. Steve's works is only about two miles from me and I can take a copy over to him.

 

[LoneStar]

Lucas made a model KNV magneto (42052) for a 50-degree V-twin, which used a face cam. It's listed for 1938-39 Matchless 1000cc twins. Probably not too many around...

 

[Chris Launders]

Yes when building the MO1/MSL hybrids I needed to fit the appropriate rotation slip ring and machine the 2nd pickup hole.

There is an article with I think all the information of modifying a face cam in a Brough Club publication I have, I will dig it out tonight and review it, if it's of any use we will have to work out a way of getting a copy to you (the forum won't allow me to send you a private message, I've tried already)

I've had a look and I think the author has it wrong, as he shows making the 2nd ramp 155 degrees after the original, this would make the 2nd spark BEFORE the point of maximum flux, and as already discussed it needs to be AFTER. so it actually needs to be 205 degrees (or 210 for a 60 degree engine), the existing pickup would then be the rear cylinder.

Other than that there is little information of use other than modifying the slip ring yourself.

 

[oexing]

Sorry for spoiling the story, but I cannot see what difference face cams could achieve as opposed to common slip rings. This will not achieve a better spark at the "pole position" that does not fit a v-twin at the slightly offset best flux point when having the common 180 degrees type armature. I did a test last night with my stock single mags and manual a/r on them. Definitely the best spark is shown at retard - logically you want best spark when kicking. With going to advanced sparks appear a bit weaker but would not matter in a running engine with higher speeds.
Certainly with v-twin mags you have to see the whole picture for the other spark plug as well and here I am not quite clear what´s best. You have to have some compromise for getting acceptable sparks from both leads. So I will do some more tests with best flux on the 205 deg position or maybe the 155 deg position - not quite clear to me yet.
Anyway, advanced or retarded is no factor as ATD is in the drive gear so flux positions are unaffected - but by now I have learned why they had the ATD in the drive gear for v-twins and not at the breaker end: The internal timing would be destroyed for one cylinder with a 180 degrees armature and common mag as it is handcapped by design already on a v-twin.
I converted 4 cylinder Vertex mags for the v-twin , certainly modded one breaker cam for 205 degrees but had to keep all 4 cams for getting sparks at all. So there will be two waisted sparks under the distributor cap going onto two brass plates for earth but the other active sparks are fat enough in all conditions so experimenting on them is easy, the cams sit on a plain taper so any positions of flux can be tried for best effect on both plugs. This is no headache as the Vertexes show impressive sparks anyway, even spinning them reverse.

Vic

 

[Chris Launders]

Sorry for spoiling the story, but I cannot see what difference face cams could achieve as opposed to common slip rings. This will not achieve a better spark at the "pole position" that does not fit a v-twin at the slightly offset best flux point when having the common 180 degrees type armature. I did a test last night with my stock single mags and manual a/r on them. Definitely the best spark is shown at retard - logically you want best spark when kicking. With going to advanced sparks appear a bit weaker but would not matter in a running engine with higher speeds.
Certainly with v-twin mags you have to see the whole picture for the other spark plug as well and here I am not quite clear what´s best. You have to have some compromise for getting acceptable sparks from both leads. So I will do some more tests with best flux on the 205 deg position or maybe the 155 deg position - not quite clear to me yet.
Anyway, advanced or retarded is no factor as ATD is in the drive gear so flux positions are unaffected - but by now I have learned why they had the ATD in the drive gear for v-twins and not at the breaker end: The internal timing would be destroyed for one cylinder with a 180 degrees armature and common mag as it is handcapped by design already on a v-twin.
I converted 4 cylinder Vertex mags for the v-twin , certainly modded one breaker cam for 205 degrees but had to keep all 4 cams for getting sparks at all. So there will be two waisted sparks under the distributor cap going onto two brass plates for earth but the other active sparks are fat enough in all conditions so experimenting on them is easy, the cams sit on a plain taper so any positions of flux can be tried for best effect on both plugs. This is no headache as the Vertexes show impressive sparks anyway, even spinning them reverse.

Vic
View attachment 61966

View attachment 61967

My question about face cams was nothing to do with magnetos for a Vincent or the efficiency of them (which is why I asked Steve directly and included "on a slightly different subject") rather that as the ring cam ends for Lucas magdynos (as fitted to my Brough Superiors) are getting scarce and expensive had he come across the face cam version and if he knew a source of the V twin face cams, as converting a face cam single magdyno would currently be easier and much cheaper than sourcing the parts to do a ring cam conversion if V twin face cams were available.

 

[fogrider]

Whilst on this fascinating discussion about magneto's, ( mostly beyond me ), I have 3 KVF's but on all 3 the cam rings are duff in that the spark timing is not in synch with the 50 deg engine. Two appear to have been hand ground to true them - badly. I have not yet been able to find anyone making new cam rings so it seems pointless lashing out to have have a mag rebuilt.
Question- is , is anyone making new cam rings for the KVF ?

 

[oexing]

Thanks, Chris, I know nothing about these face cam types, appear quite exotic to me and did not know about scarcity of common cam rings. So I was expecting some interesting features with face cam types. But when it comes to providing cam rings they are quite easy to manufacture I´d think. You don´t need a CNC for them , just a dividing head which typically has 360 degrees graduation on them. So producing cams exactly with desired ignition ramps is simple to do - thinking of Norman, he may be willing to find a company for this - to keep him busy in retirement . . . .
I will have a look into the wrecked Lucas mag I got with the B Rap bits for a better understanding.

Vic

 

[timetraveller]

Retirement eh Vic! Is that what it is. I've just spent all morning trying to design a few different options for the lower mounting on a new rear coil over damper. Last weeks visit to a friend with a fully assembled bike threw up some problems with the present set up hence the need for design work. I'm trying to keep it so that it looks good as well as just working. We will see.
I do not think that it would be difficult to find someone to make cam rings but with CNC work no one is going to want to write a program to make a couple. Perhaps someone who is into magnetos would like to do a bit of research as to what numbers are required.

 

[Cyborg]

You could copy a known good cam, but the drawings for cam rings are out there… at least they are for the KVFTT, which would give you most of the information. They give the location and radius for the ramps etc. Lucas machined them leaving an allowance for grinding. Prior to grinding they were annealed, case hardened, tempered… to avoid distortion they say to hold the cam perpendicular to the surface when performing both heating and the hardening operation…. so all together a bit of work. I have a new KVF cam ring which was apparently made in California (assuming I recollect correctly), so maybe there are more floating around. I could ask around if you wish.