Misc: Ignition Lucas KVF Magneto on a Rapide with manual adv./ret.

[MichaelOrgzey]

Hi everybody!
I recently came across a picture of the forementioned magneto on a Rapide with manual advance. Has that ever been used by the works at all?
I was under the impression that it doesn't work well on a V-Twin due to problems with the internal timing of the mag and causing problems starting.
Any information or opinions would be appreciated!
Cheers, Michael

 

[Robert Watson]

Pre war twins used a base mount mag with manual advance retard and only a couple of degrees different on the V twin. The only issue with a manual system for street use is remembering to change it for a decent idle when sitting in traffic. I put a Modern BT-H on the A twin.

 

[Peter Holmes]

I am sure there are for better qualified people than I to answer this question, but here goes, there is no internal timing within the magneto. The manual advance and retard is achieved by moving the slip ring in relation to the heal cam of the points. So as long as the slip ring is moved by the cable by the same amount that the ATD does, then there is no difference between the fully advanced and fully retarded position, the only possible difference is with the advance curve, in theory the ATD should give a graduated advance, the manual lever, as the name suggests is in the hands of the rider.
There are folks that swear by a manual magneto, my good friend Marcus Bowden being one of them, he rides by his senses, interpreting just where the engine happiest under certain conditions, load etc. Personally I am far to lazy, I had a 1928 AJS with a manual mag, and it drove me crazy, always fiddling with the lever trying to achieve the best performance.

 

[Chris Launders]

I'm sorry Peter but there is internal timing in the magneto, one cylinder (usually the rear) fires in the optimal position fully advanced and the other cylinder fires (in the case of Vincent's, 25 degrees) away from optimal. In the case of a magneto with an auto advance it stays this way all the time as it adjusts the timing BEFORE the magneto, but with a manual advance and retard you move the timing away from this position as the armature stays fixed in relation to the engine but you effectively move the points backwards when retarding, weakening the spark to both cylinders to some extent especial the 2nd cylinder which was already below optimal.

 

[Bill Thomas]

I raced for years with a manual mag.
Huge mistake.
Should have used a D Distributer.

 

[Peter Holmes]

I'm sorry Peter but there is internal timing in the magneto, one cylinder (usually the rear) fires in the optimal position fully advanced and the other cylinder fires (in the case of Vincent's, 25 degrees) away from optimal. In the case of a magneto with an auto advance it stays this way all the time as it adjusts the timing BEFORE the magneto, but with a manual advance and retard you move the timing away from this position as the armature stays fixed in relation to the engine but you effectively move the points backwards when retarding, weakening the spark to both cylinders to some extent especial the 2nd cylinder which was already below optimal.

Well I did say there were people on this forum far more qualified than myself to answer the original question, I still struggle to understand what you are saying, but I guess it is all to do with the optimum position for maximum flux. When I had a magneto converted years ago from ATD operation to manual, they simply changed the end casting of the magneto and a slip ring with a slot it to accept a large type nipple on the end of the cable, that was all, so instead of moving the magneto shaft, the slip ring moved around the magneto shaft, if that makes sense, nothing was changed internally inside the magneto, but I am well out of my depth here.

 

[MichaelOrgzey]

Thanks for your replies, I'm no expert at all but I think Chris is spot on with the technical background. I'm wondering if all the post war twins where originally equipped with Lucas KVFs with ATDs or if manual advance was an option.

 

[MichaelOrgzey]

Peter, I assume you're talking about a Lucas KVF you had converted. Why did you decide to have it done and did it have any impact on Starting at all?

 

[Chris Launders]

Hi Peter, yes what you say is correct nothing changes inside but the armature has an optimal sparking position and one cylinder is on it and the other isn't. But by moving the advance/retard from before the armature to effectively after it you worsen the situation.
So say the auto unit moves 15 degrees the optimal spark is in the same place whether advanced or retarded.
But with a manual adv/ret you move the spark away from it's optimal position by 15 degrees, the real problem is the other spark WAS 25 degrees off optimal already and now you have moved it a FURTHER 15.

 

[Chris Launders]

I would have thought as with anything else a manual magneto would be an option.

If they weren't where have they all come from as the manual end section is the opposite hand to most bikes other than Vincents.