magneto

[RDfella]

Here’s a query for the magneto geniuses.
I have a Lucas KVF on a 60 degree vee twin. Trouble is, the manual advance comes out at a very awkward angle, so I’d like to rotate the cam ring / advance holder through 180 degrees. Were this a parallel twin, I’d not see a problem but, being a 60 degree engine, I’m concerned that doing this might have the points opening out of kilter with the armature.
Any thoughts?
 

[muskrat]

G'day RD.
I'm probably the least knowledgeable with magies but can you rotate the whole mag 120 degrees by the flange.
Cheers

[groily]

RD - you can rotate the camring and not have anything out of kilter - you just need to reverse the HT leads as well.
The rear cylinder on an engine whose crank turns clockwise from the timing side will be the one fired after the shorter period of 'points closed', the front one after the longer interval (as that's the one that is running 'late' at the mag). The firing interval is 150:210°, or should be if the camring is a 60° one.

Manual control on a 60° V engine means the mag has to be in super good shape. In the advanced position the second cylinder is already running 30° retarded from the ideal firing point, and with additional retard applied, that second spark will become appreciably weaker still. (You can't compromise the setting by asking one cylinder to fire 'early' because it won't, as the HT coil won't be charged up. But you DO want to get that first spark to arrive as early as possible, ie with the camring as 'advanced' as you can safely get it in its housing; that way the delay on the second lobe is minimised.

Couple of supplementary things:
Traditionally, the second, weaker spark on a V is arranged to be the negative one (spark goes from centre electrode to earth on plug), so reversal of the magnetism is a good move if possible. (Doesn't make THAT much odds, but every little helps.)
Also it's very worth checking the HT brushes' position on the brass strip of the slipring at full advance and full retard on both cylinders with the points just opening- to be quite sure the brush is always fully on the strip at the firing points. The typical slipring on a KVF is stretched, with manual advance/retard, on a 50° application, so this isn't a trivial point. I haven't tried a KVF at 60°, but I can say that a manual mag from an HRD needed every millimeter of the brass strip on the ring to cover the range. It might just  be that a reduction in camring movement to limit retard would be necessary, because if the spark flew partly  across plastic or bakelite, trouble wouldn't be far away!

Have to say, my heart sits in my mouth when testing wide-angle V-twin mags, as the requirement to get that second spark at reasonable rpm is pretty important for easy starting. On a 50° V, the difference in rpm required to promote the 1st and 2nd spark would typically be about 30 or 40 rpm, with the first spark (5.5mm Lucas-spec test gap) delivered (100% continuously) at under 130 rpm (at best). That's 260 crank rpm rising to 320-340, say. Doesn't sound a huge lot - but in kickstart terms  . . .

[Greybeard]

Groily, you are a font of knowledge!

[groily]

Not at all sure about that GB. A little learning and all that!! Dangerous or what?!
But  . . . V twins are fun to play with, and they do offer some interesting twists because no magneto was ever really designed to function assymetrically. Demanding they do so means you're always starting from a compromised position. The wider the angle, the more compromised. Bosch did some good stuff on 'staggered' armature design to try to compensate for assymetry on Vs, and other folk did similar.  Lucas though, with the KVF, basically just asked a K2F to do its best, with minor tweaks. And they work surprisingly well for all that, it does have to be said.

[RDfella]

Thanks for that folks.
Musky – if only I could! The mag is mounted vertically and unfortunately rotating 1/3 turn clockwise or anti-clock ends up with the cable pointing away from the bike; the other way puts the adv/ret pointing straight at the rear cylinder – hence my wish to rotate the cam ring and adv/ret assy by 180 degrees (which is possible provided I drill & tap one mounting hole) to get it pointing rearward.
Bill – lots of good info there. Have tried to wrap my head around this but got brain fade. Seems to me if I rotate the cam ring 180 degrees, on a parallel twin I’d merely have to swap plug leads, but on a 60 degree there’s no cylinder firing 180 degrees later (or earlier) so I’d need to move the timing 30 degrees forward or back. Which is where I start getting brain fade trying to figure whether that means points won’t be opening at the most appropriate time in relation to the armature.
Or is it that, as you say, that the mag was designed for 180 degree firing and in that case I’d merely be swapping over the cylinder likely to get the weaker spark with its partner – albeit having to retime the ignition in relation to the engine?
I had the mag rebuilt and converted to 60 degree when I started this project, but that was a few years back and I can’t remember who did it. Probably DH Day. It hasn’t been used since then and it’s only now that I’m trying to finish off the project (before I get to old to do so!) that I'm finding these issues.
Having said all that, I have a similar mag on a 50 degree vee twin. Works great, but it’s clear that on starting one cylinder is more enthusiastic than the other one. Often wondered why Brough etc used twin mags on their vee twins.

[chaterlea25]

Hi RD and All,
Last year I was working on a 23 HD model F whiich has mag ignition, It has a Bosch magneto
However the cam and end assembly with the advance lever were on the "wrong" side of the mag to work with the linkage from the A/R twistgrip
The cam ring is held in the moving part by screws, I  undid  the screws and rotated the moving part 180 degrees leaving the cam ring in its original position, cutting a new slot for the A/R limit peg and drilling new holes for the cam screws

So with the KVF would it be possible to cut new slots in the cam ring for the A/R plunger and limit peg 180 degrees away from present position, then rotate the housing 1/2 turn
hopefully then keeping the sparks jumping in the correct direction

 
John

[groily]

Tis very possible John. Renotching and moving camrings is always an option, as is modifying the cb assembly to reposition the integral keyway (to get the points to open at the right moment with the camring or housing in an altered position). Both these things have to be done regularly when battling with some of these old beasts. (New Lucas camrings usually come blank, so you can grind yer notches wherever you want.)

What matters on a V is that at full advance the points just start to open a few degrees after a flip point for the 'advanced' cylinder. The trailing cylinder will then be correct if the camring is the right one. (Always ensuring the required rotation of the mag has been borne in mind!)

A good example of variations in camring and AR cable position in practice is between, say, the BTH on a Rudge (with AR cable coming off almost horizontally) and the same mag on a Velo, with the end housing 90° differently-placed to have the cable coming off vertically. Same basic instrument but the camring is set differently in the end housing. (And BTHs are a bit harder to modify  than Lucas, but that's another story.)

Reversing a symmetrical camring by 180° on a  parallel twin makes no odds to which plug lead goes where RD. But on a V with an assymetric ring, you gotta get the sparks up there in the correct sequence. It's a one-way-only thing.
If the mag you've got there has been modded, I hope the HT brushes stay on the brass of the slipring in all positions of the AR lever/camring. It's a fine line sometimes between having enough arc of brass to support the AR movement, and not having so much that the thing tries to fire both cylinders at once because both brushes are in contact in certain positions. So yes, twin mags per Brough can be good - where there's the room!

[chaterlea25]

Hi Bill, and All,

Been there and done that on the Rudge BTH  ! In my defense it was a very long time ago
But it still happens, a friend got the BTH for his Rudge back from rebuild a while ago and the A/R was vertical so he undid the end screws and turned the housing, then wondered where the sparks had gone to 

I have a V twin ML mag where the pickups are angled, I,m presuming this is so the will line up with the slipring brass segment better than at 180 degrees?

John

[groily]

I have a V twin ML mag where the pickups are angled, I,m presuming this is so the will line up with the slipring brass segment better than at 180 degrees?

Yup - there are a few V mags like that John, where the HT pick-ups are offset. And I'm sure you're right as to 'why'.

[RDfella]

This morning I thought I’d celebrate my birthday by getting to grips with this mag issue. Apart from fitting the occasional bearing in a BTH (and struggling with those insulating gaskets) I’ve tended to leave building mags to those who do it regularly. Which is why I don’t fully understand what I’m doing here. I realise the armature has to be timed with the points, but don’t know how you locate max flux – or, indeed, where it should occur.
Anyway, I took the mag off and job no 1 was to verify that at both adv and ret both slip rings were under the brushes. So far so good. I then marked the case as to where the drive gear was for each points opening. I then reversed the cam ring holder and checked again. I noticed that the asymmetry had swapped sides. The front cylinder points were now opening later than previously (bearing in mind that they were rear cylinder originally) and the rear cylinder earlier (again, bearing in mind these were formerly the front cyl points).  I think.  Which presumably is correct. Designing cams was easier than this!
My next plan is to test the mag for spark at each pickup, which will presumably verify whether the change is do-able. If that’s OK, I’ll go ahead and drill / tap the 3rd hole and refit / retime the mag to the motor.
And if anyone’s wondering why I’m discussing vee twins on this forum, well, it is in a ’62 Golden Flash frame.

[berger]

o yeah I just love those bearing insulators , I had to make a sleeve up to keep bearing away from magnet power. a bit fiddly diddly but got it in ok

[groily]

Happy Birthday RD!
Sounds as if you're getting there, no sweat. It will work, it really will.
Very good news that the brushes are on the brass in all positions of A/R - that's one headache you won't have - and it's a great project by the sounds of it.  Cheers!

[RDfella]

One step forward, one back..... Will get there eventually though. Good blue spark, about 5/16 long from rear cyl pickup, weaker one from front, as expected. Fitted mag and timed it - or tried to. When the rear cyl is timed (approx 30 deg adv) the front is nowhere right - almost 50 deg of advance. Although the mag is marked KVF 60, I'm wondering if the cam ring is actually a 50. And the points gap is greater on the front compared with the rear (.020 / .012) which doesn't help either. So it looks like I'm in need of a decent 60 deg cam ring. What fun bikes are.

[chaterlea25]

Hi RD
It probably the end housing is not concentric when turned around, if you follow me
Put the end housing back and check the points gap again and measure the firing intervals
You probably will have to re centre the end housing

John