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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: 178890" data-attributes="member: 5866"><p>I agree, that's what you would want in an ideal world. Sadly the physics of the way in which a spark is generated in a magneto means it cannot be done! Let me try to explain why. Imagine a clock face where the 12 o'clock position is where the magnetic break occurs. As soon as the break does occur, the magnetic field starts to rapidly collapse. The best spark can be generated at that point - let's say one minute past. As the ignition is manually retarded, the points open later. Lets say fully retarded is at 11 minutes past - that gives us a ten minute span. The best spark occurs when fully advanced at 1 minute past, the weakest spark occurs when fully retarded at 11 minutes past. Now lets rejig everything so that the ten minute span is moved backwards by ten minutes. move the lever to the fully retarded position - that will now be at 1 minute past and you will get the best possible spark - ideal. Now move the lever to advance the ignition timing. Almost immediately, the points will now be opening before the magnetic break at 12 o'clock. The magnetic field will not have started to collapse and so there will be no spark generated. Move the lever to the fully advanced position at 9 minutes to 12 and you'll be even further away from the point where a spark can be generated. Yes you'll have the best spark with the lever at the fully retarded position but you won't get any sparks anywhere else on the advance/retard range!</p><p>This is all about the internal timing of the magneto (not the same as the ignition timing of the engine). There's more about all this here: <a href="https://www.themagnetoguys.co.uk/magneto-internal-timing" target="_blank">https://www.themagnetoguys.co.uk/magneto-internal-timing</a> and here: <a href="https://www.themagnetoguys.co.uk/advance-and-retard" target="_blank">https://www.themagnetoguys.co.uk/advance-and-retard</a></p><p>and there's some info about V twin magnetos and also using Lucas KVFs on Vincents here: <a href="https://www.themagnetoguys.co.uk/v-twin-magnetos" target="_blank">https://www.themagnetoguys.co.uk/v-twin-magnetos</a></p><p>Incidentally, the Lucas technical data shows four variations of the KVF. All of them are clockwise rotation and none of them have a manual advance and retard fitted. As already pointed out by others in this thread, one of the sparks on a KVF is already weak because it is occurs late after the start of the collapse of the magnetic field. Adding a manual retard on top of that would make the situation even worse so Lucas didn't offer it as an option. There are of course KVF magnetos out there fitted with a manual advance and retard - but they didn't leave the factory like that!</p></blockquote><p></p>
[QUOTE="Steve Marks, post: 178890, member: 5866"] I agree, that's what you would want in an ideal world. Sadly the physics of the way in which a spark is generated in a magneto means it cannot be done! Let me try to explain why. Imagine a clock face where the 12 o'clock position is where the magnetic break occurs. As soon as the break does occur, the magnetic field starts to rapidly collapse. The best spark can be generated at that point - let's say one minute past. As the ignition is manually retarded, the points open later. Lets say fully retarded is at 11 minutes past - that gives us a ten minute span. The best spark occurs when fully advanced at 1 minute past, the weakest spark occurs when fully retarded at 11 minutes past. Now lets rejig everything so that the ten minute span is moved backwards by ten minutes. move the lever to the fully retarded position - that will now be at 1 minute past and you will get the best possible spark - ideal. Now move the lever to advance the ignition timing. Almost immediately, the points will now be opening before the magnetic break at 12 o'clock. The magnetic field will not have started to collapse and so there will be no spark generated. Move the lever to the fully advanced position at 9 minutes to 12 and you'll be even further away from the point where a spark can be generated. Yes you'll have the best spark with the lever at the fully retarded position but you won't get any sparks anywhere else on the advance/retard range! This is all about the internal timing of the magneto (not the same as the ignition timing of the engine). There's more about all this here: [URL]https://www.themagnetoguys.co.uk/magneto-internal-timing[/URL] and here: [URL]https://www.themagnetoguys.co.uk/advance-and-retard[/URL] and there's some info about V twin magnetos and also using Lucas KVFs on Vincents here: [URL]https://www.themagnetoguys.co.uk/v-twin-magnetos[/URL] Incidentally, the Lucas technical data shows four variations of the KVF. All of them are clockwise rotation and none of them have a manual advance and retard fitted. As already pointed out by others in this thread, one of the sparks on a KVF is already weak because it is occurs late after the start of the collapse of the magnetic field. Adding a manual retard on top of that would make the situation even worse so Lucas didn't offer it as an option. There are of course KVF magnetos out there fitted with a manual advance and retard - but they didn't leave the factory like that! [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Lucas KVF Rebuild - UK Recommendations?
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