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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Lucas KVF Rebuild - UK Recommendations?
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<blockquote data-quote="Steve Marks" data-source="post: 179017" data-attributes="member: 5866"><p>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: <a href="https://www.themagnetoguys.co.uk/reversing-direction---magnetos" target="_blank">https://www.themagnetoguys.co.uk/reversing-direction---magnetos</a></p><p>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: <a href="https://www.themagnetoguys.co.uk/voltage-at-the-spark-plug" target="_blank">https://www.themagnetoguys.co.uk/voltage-at-the-spark-plug</a>. Hope this answers some of your questions.</p></blockquote><p></p>
[QUOTE="Steve Marks, post: 179017, member: 5866"] 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: [URL]https://www.themagnetoguys.co.uk/reversing-direction---magnetos[/URL] 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: [URL]https://www.themagnetoguys.co.uk/voltage-at-the-spark-plug[/URL]. Hope this answers some of your questions. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Lucas KVF Rebuild - UK Recommendations?
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