Misc: Ignition Lucas KVF Rebuild - UK Recommendations?

[Cyborg]

You also want to check the polarity so the front (weaker) cylinder gets a little more help….. and it is definitely the front that gets the weaker spark. When retarded, it is way past the flip point …. flux reversal …or whatever you want to call it.
This is a different mag body than the KVF, but there are manual advance end housings out there that will bolt onto the KVF. The slack wire advance ones are very rare… at least over here they are.

This is the setup I used to cut the slots for the manual adv/ret linkage. The stone in the foreground was reshaped with a diamond dresser so it would create the correct V shaped notch for the adv/ret link.

 

[oexing]

Indeed, it seems the internal timing in the mag has to be modded for maximum voltage on that cylinder you use for kicking the beast. I have no idea how polarity of magnets acts on sparks, polarity changes in rotation all the time on twin mags when north and south fly by and the breaker lifts, just that for one cylinder on a v-twin you will get a poorer spark - not a matter when it is not the one you choose for kicking.

Vic

 

[Chris Launders]

Indeed, it seems the internal timing in the mag has to be modded for maximum voltage on that cylinder you use for kicking the beast. I have no idea how polarity of magnets acts on sparks, polarity changes in rotation all the time on twin mags when north and south fly by and the breaker lifts, just that for one cylinder on a v-twin you will get a poorer spark - not a matter when it is not the one you choose for kicking.

Vic

You can't really re-arrange the firing to give the best spark to the front cylinder, if you did it would mean the rear spark firing before maximum flux, and that doesn't work, it needs the full time under the magnet to build up flux whereas it decays relatively slowly after passing the magnet.

I have tried this by mistake converting my first single cylinder MO1 into a V-twin one.

The best solution is one friend was working on (he may still be, I haven't spoken to him lately) which is connecting two SR1 magnetos together but 25 degrees out of phase, so it is in effect a two separate magnetos in one unit.

 

[bmetcalf]

I suppose a flange mount SR2 rotating magnet mag with a 50 deg cam would have the same low flux problem on one cylinder. On the other hand, that is basically what the Hunt mag is on Alp's Bonneville 666 and it works pretty well!.

 

[Cyborg]

Indeed, it seems the internal timing in the mag has to be modded for maximum voltage on that cylinder you use for kicking the beast. I have no idea how polarity of magnets acts on sparks, polarity changes in rotation all the time on twin mags when north and south fly by and the breaker lifts, just that for one cylinder on a v-twin you will get a poorer spark - not a matter when it is not the one you choose for kicking.

Vic

Voltage generated by the coil is alternatively positive and negative. One plug gets negative and the other positive. It is easier for the spark to jump from a hot centre electrode to the plug body. This occurs when it gets the negative voltage, so you would want the front cylinder to get the negative…. The KVF mag is probably set up that way, but I’m not sure… regardless, it could have been inadvertently switched when it was remagnitized at some point. Just for the record, I’m not an expert on this stuff, so best you do your own research. The only credential I have is the magneto pictured above which has a nice fat spark at kicking speeds.

 

[Cyborg]

I suppose a flange mount SR2 rotating magnet mag with a 50 deg cam would have the same low flux problem on one cylinder. On the other hand, that is basically what the Hunt mag is on Alp's Bonneville 666 and it works pretty well!.

I have no idea what a SR2 is, but as far as I know the Hunt mag utilizes rare earth magnets, so that would give it a leg up.

One of the items on my wish list is seeing Alp’s engine on a dyno set up in a completely dark room…. just so I could film the lightning bolts shooting out between the head and barrel.…. also film it with a strobe, so you could see how much everything flails around.

 

[oexing]

The Lucas SR type is the supposedly superior rotating magnet type, post war, but no a/r device on it. And still I do not believe in the religion about positive or negative sparks to be vastly different, on any twins with mags you get changed polarities all the time for the plugs. Only singles can have same pole on the single plug from the breaker with only one cam. So converting a single mag for a twin is possible as the magnet inside is the same . Certainly having it on a v-twin will be same trouble like with the original, the rotor/armature is not asymmetric typically for the 50 degree twin mags I guess.
I am not clear what it means when shifting max. flux to the front pot. Sure the situation will not be optimum for the rear pot, would think it will be similar to present state with the front pot ?? With a manual timing system I´d think the cam race is an extra component that could be set in a better position in its housing that has the Bowden wire. Would that help getting the firing point to the position of max flux for the desired pot? No clue at the PC at this time . . . . . .
For my B Rapides I did a complicated mod using the VW Vertex 4 cyl magnetos. Fat sparks certainly, does not matter much if spun one way or other. Same dilemma, you can not get same max flux when modding it for a v-twin but the mag is that strong it does not matter. Below a YT clip I did with the Vertex.

Vic
YT Vertex:
Vertex

 

[Chris Launders]

Cyborg, the SR magneto is usually base mounted but can easily be altered to a flange mount and is almost identical to a Hunt magneto (I have two Triumphs, one with a Hunt mag and one with an SR2).

Oexing, SRs were available with manual A/R often used on earlier Manx Nortons, but this moves the points thus going away from optimal flux. You can of course use an ATD on either the Hunt or SR

The Hunt or similar Morris are probably the best magnetos if you don't want the electronic BTH, as they have had decades more development and spares are currently available.

 

[stu spalding]

I suppose a flange mount SR2 rotating magnet mag with a 50 deg cam would have the same low flux problem on one cylinder.

Hi Bruce, the SR1 mag has the points of maximum flux density at 155/205 degrees, so it would be ideal if converted to V twin use. Cheers, Stu.

 

[bmetcalf]

The SR1 only has one spark plug lead.