Misc: Ignition Lucas KVF Rebuild - UK Recommendations?

[Steve Marks]

Some interesting posts here which I may be able to add something to.
As already mentioned, when the points are closed, the heel of the cam is moving 'in free air' - it is not touching the cam ring. The points open when the fibre heel hits the rising edge of the cam. Whereabouts the heel hits the rising edge will depend on the size of the points gap. The size of the points gap is adjusted when the heel is on the top of the ramp. A small points gap will mean that the heel hits the ramp near the top. If the gap is increased, the heel will be hitting the ramp earlier. Changing the points gap changes the internal timing of the magneto - covered in more detail here: https://www.themagnetoguys.co.uk/points-gap
It's important to have the internal timing right - for all magnetos not just V twin versions. That's what the eccentric screw is for on the Lucas KVFs. A slot in the bottom of the cam ring fits over the eccentric screw so that, as the screw is turned, the cam rotates slightly thereby allowing the internal timing to be adjusted. In a fixed ignition magneto, the width of the slot in the cam ring is a clearance fit over the head of the screw. On manual ignition cam rings the width of the slot is much wider. The wider slot allows the cam ring to be rotated by the manual control to adjust the engine ignition timing. Unfortunately this also adjusts the internal timing of the magneto as mentioned earlier in this thread. One side of the wider slot is hard against the eccentric screw when the cam ring is in the fully advanced position, and against the other side of the slot in the fully retarded position. As before, the eccentric screw is adjusted to give best internal timing when the cam ring is in the fully advanced position.
There was some discussion early on in this post about setting the internal timing to give the best spark at the retarded position. I tried to explain why this could not be done but seems there are some who cannot see that. There are a couple of things I can mention which reinforces what I was trying to say. The attached picture shows a Lucas K2F being tested on one of our test benches. The test bench allows us to control the speed of the magneto from zero to about 5,000 rpm. I think we are all agreed that a magneto finds it easier to produce sparks as the speed increases. That's why we are interesteed to see how slow the speed can go and yet still get a reliable spark. So we turn the speed right down until we see a missing spark. Then speed it up a fraction to get back to regular reliable sparks. This is all done with the magneto in the manually fully advanced position. If we now move the manual control to fully retarded, the sparks disappear - advance the ignition and they come back. When the manual control on these magnetos retards the ignition timing by rotating the cam ring, it also retards the internal timing of the magneto - that's why the sparks are lost. Testing documentation from Lucas states what the criteria needs to be for testing their magnetos. The part we are concerned with is the low speed test. Lucas states that the HT leads should be connected to independent 3 point test gaps set to 5.5mm. The lowest speeds at which a maximum of 5% missing sparks is noted is given as two figures - one when fully advanced, the other when fully retarded. On all the magneto types listed, the fully advanced speed is lower than the fully retarded speed. For the K2F, the figures are 150rpm advanced, 180rpm retarded. This, I believe, reinforces the idea that sparks are weaker when retarded, so much so that the speed of rotation needs to be increased to achieve acceptable performance. Yes, agreed, only 30rpm which, as a percentage, is not much. Interestingly, although the KVF magnetos we have been discussing were only provided with fixed ignition, they do none the less provide two figures. 150rpm (as with the K2F) when fully advanced, 250rpm when fully retarded. The fact that the KVF has one (internally) retarded spark even when fully (manually) advanced, is highlighted here by the need to run at 250rpm to get reliable sparks when retarded.

 

[Steve Marks]

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.

I've found the article! Yes Chris you are correct - the second (new) ramp needs to be 205 degrees on from the first (original) ramp. An easy mistake to make and probably something that was noted and corrected in following magazines. Trouble is, once these things are printed and published, they can often be taken as 'gospel' Well spotted!

 

[Chris Launders]

The Brough Superior Club spares stocks 50 and 60 degree cam rings in both clockwise and anti-clockwise for the pre-war magnetos so somebody must be making them, the only difference is the "back" where the locating slots etc are.

Don't bother trying to contact the spares man to enquire, he's just gone to Australia for a month or so, taking in the Tassie Tour on a Brough

 

[greg brillus]

Are we talking Mike Smith, Howard Wilcox, and Nick Jeffery's........All great blokes......Nick is a Vincent man......

 

[Chris Launders]

Mike Smith is the spares man.
When I decided to build my SS100 replica I contacted Howard, and never having met me he allowed me loose at his place to measure/photograph anything I wanted, when back home I realised I had missed a measurement I needed (yes I made my own) on the forks he offered to bring me a set to measure the next week as he was in the area, I assured him the just the measurement would be fine.

 

[Cyborg]

Some interesting posts here which I may be able to add something to.
As already mentioned, when the points are closed, the heel of the cam is moving 'in free air' - it is not touching the cam ring. The points open when the fibre heel hits the rising edge of the cam. Whereabouts the heel hits the rising edge will depend on the size of the points gap. The size of the points gap is adjusted when the heel is on the top of the ramp. A small points gap will mean that the heel hits the ramp near the top. If the gap is increased, the heel will be hitting the ramp earlier. Changing the points gap changes the internal timing of the magneto - covered in more detail here: https://www.themagnetoguys.co.uk/points-gap
It's important to have the internal timing right - for all magnetos not just V twin versions. That's what the eccentric screw is for on the Lucas KVFs. A slot in the bottom of the cam ring fits over the eccentric screw so that, as the screw is turned, the cam rotates slightly thereby allowing the internal timing to be adjusted. In a fixed ignition magneto, the width of the slot in the cam ring is a clearance fit over the head of the screw. On manual ignition cam rings the width of the slot is much wider. The wider slot allows the cam ring to be rotated by the manual control to adjust the engine ignition timing. Unfortunately this also adjusts the internal timing of the magneto as mentioned earlier in this thread. One side of the wider slot is hard against the eccentric screw when the cam ring is in the fully advanced position, and against the other side of the slot in the fully retarded position. As before, the eccentric screw is adjusted to give best internal timing when the cam ring is in the fully advanced position.
There was some discussion early on in this post about setting the internal timing to give the best spark at the retarded position. I tried to explain why this could not be done but seems there are some who cannot see that. There are a couple of things I can mention which reinforces what I was trying to say. The attached picture shows a Lucas K2F being tested on one of our test benches. The test bench allows us to control the speed of the magneto from zero to about 5,000 rpm. I think we are all agreed that a magneto finds it easier to produce sparks as the speed increases. That's why we are interesteed to see how slow the speed can go and yet still get a reliable spark. So we turn the speed right down until we see a missing spark. Then speed it up a fraction to get back to regular reliable sparks. This is all done with the magneto in the manually fully advanced position. If we now move the manual control to fully retarded, the sparks disappear - advance the ignition and they come back. When the manual control on these magnetos retards the ignition timing by rotating the cam ring, it also retards the internal timing of the magneto - that's why the sparks are lost. Testing documentation from Lucas states what the criteria needs to be for testing their magnetos. The part we are concerned with is the low speed test. Lucas states that the HT leads should be connected to independent 3 point test gaps set to 5.5mm. The lowest speeds at which a maximum of 5% missing sparks is noted is given as two figures - one when fully advanced, the other when fully retarded. On all the magneto types listed, the fully advanced speed is lower than the fully retarded speed. For the K2F, the figures are 150rpm advanced, 180rpm retarded. This, I believe, reinforces the idea that sparks are weaker when retarded, so much so that the speed of rotation needs to be increased to achieve acceptable performance. Yes, agreed, only 30rpm which, as a percentage, is not much. Interestingly, although the KVF magnetos we have been discussing were only provided with fixed ignition, they do none the less provide two figures. 150rpm (as with the K2F) when fully advanced, 250rpm when fully retarded. The fact that the KVF has one (internally) retarded spark even when fully (manually) advanced, is highlighted here by the need to run at 250rpm to get reliable sparks when retarded.

Thanks for posting that. The eccentric grub screw still remains a bit of a mystery to me. A fellow who posts on here occasionally, checked the internal timing on a single cylinder magneto and found that it was about 16 degrees after the flip point (if I remember correctly). The magneto was functioning properly and didn’t seem to be bothered by the late timing. When you adjust the grub screw, how do you determine the best spot for it? I’m assuming close to the flip point, but if a mag works at 16 degrees, they don’t seem that fussy. Do you measure the spark voltage or just use the spark tester to determine the best position? If you were setting the internal timing on a KVF or KVFTT , how many degrees after the flip point would you set number one cylinder?

 

[oexing]

Maybe with some different wording I can better post my views on the matter of internal timing and why best sparks normally are to be had at retarded:
Do we agree the most power can be produced at maximum "stretch" of magnetic lines past the iron poles just before polarity changes to next other pole ?
So this is what we want for kicking the beast at retarded, certainly to be safe from kickback ?
In my photo below that is exactly how Bosch did it with the very last and only postwar magneto in my country installed in the Earles fork BMWs which got 180 degree mags like the Brit parallel twins. The v-mark on the brass case has to be aligned with the mark on the rotating magnet and this is the retarded position. So rotation sense is anti clockwise in this view, certainly clockwise seen from a at camshaft end where the mag is mounted. So you can see , at the retarded position the field lines are stretched to the maximum amount before the polarity would change.

Now with the ATD on the breaker end the timing gets shifted to "early/advanced " for a lot less "stretch" as obviously the ignition happens earlier and so less stretch is seen from 15 degrees less rotation of rotor at advance.
This is no problem as with a running engine any slightly weakened spark from less stretch is made up by the faster speed and sparks will be powerful in the whole range of ignition curve. In photo the position of advanced, with less stretch, 15 degrees less rotation of magnet:

So with the v-twin you have to keep ignition within range of magnetic field timing for having sparks at all. When looking at the rear cylinder for no. 1 the next spark for no. 2 will be at 205 degrees from the 50 degree v twin, so more than one crank revolution. Now when wanting the best spark at maximum stretch for the rear cylinder this would mean the internal timing for the front cylinder to shift way out of range as it was already just at the limit for the rear as all of 205 degrees would have flipped polarity then.
So then only chance for a v-twin is having the rear cylinder at little stretch and the front cylinder at maximum stretch, like for retarded in other singles or 180 degree engines.
So then I cannot see any position within that range of internal timing of not producing any sparks at all. This can only happen when one of the two firing positions is way out of range. Yes, one spark might be weaker than the one closer to best position and with a Vincent engine it may be best to aim for having the engine fire at the front cylinder - and the widest rotational distance for kicking is certainly having the rear cylinder just past firing TDC by valve lifter and then big swing on the lever.
With that limitation of range on v-twins you cannot have an ATD on the breaker end or manual device at same end as this would shift the internal timing of mag way out of range for having sparks in extreme positions. So that is why the ATD has to be on the drive end as it will not effect the internal timing.
Or am I wrong again ??

Vic

 

[Steve Marks]

Thanks for posting that. The eccentric grub screw still remains a bit of a mystery to me. A fellow who posts on here occasionally, checked the internal timing on a single cylinder magneto and found that it was about 16 degrees after the flip point (if I remember correctly). The magneto was functioning properly and didn’t seem to be bothered by the late timing. When you adjust the grub screw, how do you determine the best spot for it? I’m assuming close to the flip point, but if a mag works at 16 degrees, they don’t seem that fussy. Do you measure the spark voltage or just use the spark tester to determine the best position? If you were setting the internal timing on a KVF or KVFTT , how many degrees after the flip point would you set number one cylinder?

The eccentric grub screw is used to adjust the cam so that the points open in the position of the armature which is capable of giving the best spark. If you have a manual advance and retard, that best armature position will be with the cam in the fully advanced position. So - when is the armature in the best position? If you rotate the armature gently by hand, you will notice that it appears to go ' tight' at two positions diametrically opposite each other as the armature rotates. It's not really 'tight' as in something rubbing, it's the magnet trying to 'hang onto' the armature. A few degrees more rotation and the magnet gives up, lets go and the armature turns freely again. It is just after the magnet gives up and the armature is free that you want the points to open. 16 degrees sounds quite a long way. Yes it will probably still spark, though not as easily. 16 degrees is probably close to midway between fully advanced and fully retarded. The best position we are talking about probably only a couple of degrees. It is more normally given as a distance between the edge of the armature and the edge of the laminated pole pieces. The best position varies between magnetos but is only a. few millimetres. Our American friends refer to this as the 'e-gap'. There are some example of what the measurement is and where it is measured here: https://www.themagnetoguys.co.uk/reversing-direction---magnetos
On the Lucas K1F/K2F/KVF magnetos, there is nowhere to measure this gap so has to be done by 'feel'. If you adjust so that the gap is too big, things will still work though perhaps not quite as easily as it might. Obviously that's more likely to show up in a 'tired' magneto with weak magnets. So, next question, can the gap be made too small? Well yes, it can! If it is too small, the engine will start very easily and run OK at low revs but at high revs, you will probably get a misfire. This is because, as the speed increases, the shape of the magnetic field changes. That will make the 'best' position appear a little later. The points will still open at the same position of course but that may then be before the magnetic field has started to collapse and that means no spark - hence the misfire. Measuring the spark voltage is not an easy thing to do. It comes and goes so quickly that a meter wouldn't be able to read it. Looking at it with an oscilloscope would give a better idea but things start to get a bit academic then - and not many workshops have access to an oscilloscope with a high voltage probe! Then of course you need to know what a 'good' voltage would be. That's not a simple thing to answer as there are so many factors which affect the size of the voltage needed to produce a spark. More info on that topic here: https://www.themagnetoguys.co.uk/voltage-at-the-spark-plug. Hope this answers some of your questions.

 

[Cyborg]

Thanks for the reply. I think I ended up between 8 and 10 degrees after ( on #1) and spark at kicking speed while retarded seems ok, This is on a KFC mag that I converted for the Vincent. I used a slack wire advance setup, which has caused me to become an outcast, but that’s another story. I needed to sort out the internal timing because I had to convert a KVF points cam to the KVFTT style. To find the flip point, I used a LCR meter, which was recommended by a fellow member who understands these things. Basically… the meter sends a low voltage signal through the winding and measures what comes out the other end. At least that’s how I understand it. The meter is a cheap Chinese version and its accuracy is unknown, but it doesn’t matter if the readings are off a bit because you are only looking for the highest reading.
Anyway… I’m thinking I would be better off if I advanced the internal timing some more in order to make #2 spark a little fatter when retarded ( via the lever) for starting. I wish there was some way to accurately measure that sweet spot. All I have for that is a home made 2 pole spark gap. I don’t think I’m any clearer on the eccentric grub screw. Without some sort of tool to accurately measure the output, that grub screw seems redundant to me. I wish I could talk to the fellow from Lucas who actually adjusted those things for a living.
The meter (at least this one) doesn’t respond instantly, so you can’t just turn the armature by hand. Mine was set up in the lathe so it could be turned a degree or 2 at a time. You can nail down the flip point exactly. It‘s as slick as snot on a doorknob! For locking the armature in that position, I use a set screw made of Delrin that replaces the ground brush.
That thing hanging off the side of the lathe is a DC motor from a treadmill. Just drives the chuck with a belt so I can vary the speed to what Lucas recommends.

 

[TouringGodet]

Very nice setup. Now you need to add a pressurized container around the spark plugs, to simulate the combustion chamber.