Misc: Ignition Lucas KVF Rebuild - UK Recommendations?

[Steve Marks]

I agree, that's what you would want in an ideal world. Sadly the physics of the way in which a spark is generated in a magneto means it cannot be done! Let me try to explain why. Imagine a clock face where the 12 o'clock position is where the magnetic break occurs. As soon as the break does occur, the magnetic field starts to rapidly collapse. The best spark can be generated at that point - let's say one minute past. As the ignition is manually retarded, the points open later. Lets say fully retarded is at 11 minutes past - that gives us a ten minute span. The best spark occurs when fully advanced at 1 minute past, the weakest spark occurs when fully retarded at 11 minutes past. Now lets rejig everything so that the ten minute span is moved backwards by ten minutes. move the lever to the fully retarded position - that will now be at 1 minute past and you will get the best possible spark - ideal. Now move the lever to advance the ignition timing. Almost immediately, the points will now be opening before the magnetic break at 12 o'clock. The magnetic field will not have started to collapse and so there will be no spark generated. Move the lever to the fully advanced position at 9 minutes to 12 and you'll be even further away from the point where a spark can be generated. Yes you'll have the best spark with the lever at the fully retarded position but you won't get any sparks anywhere else on the advance/retard range!
This is all about the internal timing of the magneto (not the same as the ignition timing of the engine). There's more about all this here: https://www.themagnetoguys.co.uk/magneto-internal-timing and here: https://www.themagnetoguys.co.uk/advance-and-retard
and there's some info about V twin magnetos and also using Lucas KVFs on Vincents here: https://www.themagnetoguys.co.uk/v-twin-magnetos
Incidentally, the Lucas technical data shows four variations of the KVF. All of them are clockwise rotation and none of them have a manual advance and retard fitted. As already pointed out by others in this thread, one of the sparks on a KVF is already weak because it is occurs late after the start of the collapse of the magnetic field. Adding a manual retard on top of that would make the situation even worse so Lucas didn't offer it as an option. There are of course KVF magnetos out there fitted with a manual advance and retard - but they didn't leave the factory like that!

 

[oexing]

But think about all those prewar v-twins from JAP or other types. They got manual a/r devices on their magnetos. Have you got any mag with manual timing for testing ? I bet you get a better spark at retard than at full advance. Don´t imagine you will only get sparks at best position as even before that position electric power gets generated for feeding a spark. I will do another check tonight on one of my manual mags for spark strength at advance and retard.

Vic

 

[Steve Marks]

Hi Vic. Yes I do have hundreds of magnetos here and I have set the internal timing on quite literally thousands of magnetos which I have repaired/restored over the last few decades (I do it for a living - full time!) Yes, magnetos which were designed for use on V twins do have manual advance and retard but we have been discussing the Lucas KVF which uses a body designed for use on a 180 degree twin.........

 

[oexing]

I believe the matter is having retard at best internal timing position for starting or rather advanced ? I cannot see any good argument for having it at advanced. But I check again in half an hour.

Vic

 

[Chris Launders]

I'm afraid I have to agree with Steve, you have to time a magneto with the best spark position and later, rather than the best spark position and earlier (which is what timing it with best spark at retard would be) as the voltage generated is very low on build up compared to the voltage as the field collapses (see back EMF)
I inadvertently tried this when converting my first MO1 to a V twin mag by fitting the cam ring with the front cylinder on the point of maximum flux rather than the rear. So the front cylinder fired on time and the rear early, rather than the rear on time and the front late, result, no spark on the rear.
Once I realised what was happening and relocated the cam ring with the rear on time I had sparks on both cylinders.

A friend was building the ultimate Vincent magneto I think by mating two SR1s face to face with the rotors off set by 25 degrees, so actually two magnetos in one. Unfortunately his first attempt wasn't accurately machined enough something gave way and I haven't seen him for a while to find out what progress he has made.

 

[Steve Marks]

Thank you Chris.
Exactly what Chris so accurately describes in words in his middle paragraph is explained in even more detail (waffle ) - with pictures - here: https://www.themagnetoguys.co.uk/magneto-internal-timing

 

[vibrac]

Yes I think Steve is correct
Since I have a KVF with a manual control my cunning plan is to fit an ATD and fit up the advance and retard post on the mag with a knurled disk and lock nut and find how many turns or part turns equate to a degree on the crank I will then time it for say 36 BTDC and adjust it for 34 and lock it I will then have an adjustable timing (over a short range) without all the taper and timing disc rigmarole because who knows when I will have to use E10 at a pinch or that green stained stuff I had in Spain on the Columbres rally. I can tune or If I hear a pink I can adjust on the road.

 

[Steve Marks]

That sounds like an excellent idea but (there's always a 'but' with excellent ideas ). The best spark will be when the cam ring is in it's own 'fully advanced' position. If you use your threaded adjuster to move the cam away from that position, the further you move it, the more you will retard the internal timing and, as discussed, the weaker the spark will become as a result. I'd suggest it would be best to keep your 'fine tuning' of the cam ring position as close to the fully advanced position as you can get!

 

[Chris Launders]

Thank you Chris.
Exactly what Chris so accurately describes in words in his middle paragraph is explained in even more detail (waffle ) - with pictures - here: https://www.themagnetoguys.co.uk/magneto-internal-timing

Steve, a question on a slightly different subject, have you ever come across a V twin MO1 (face cam) type magdyno, a friend had one on a V twin Matchless.

If so do you know a source for the face cams, this would be a much easier/cheaper conversion than the MO1/MSL hybrids I've built for my Broughs

 

[Steve Marks]

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.