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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Lucas K1F magneto weak spark
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<blockquote data-quote="Magnetoman" data-source="post: 138643" data-attributes="member: 2806"><p>The issue with remagnetizing is all about how hard you will have to kick the bike to start it. Even an extremely weak magnet will generate sufficient voltage for a spark, <em>if</em> the magneto spins fast enough, but that will be well past how fast someone can possibly kick it with an older "steel magnet" magneto from the pre-1930s. Alnico is more forgiving, but the effect is still present. . </p><p></p><p>A Lucas manual says 300 rpm is the low end of kick starting speeds, with 500 rpm normal. At these speeds the generated voltage will be essentially linear in speed, so if a magneto has lost half its magnetization it will require spinning it twice as fast. What this means in practical terms is that if you had a magneto that supplied sufficient voltage for starting at 300 rpm, but then remove and replace the armature for any reason resulting in it now starting at 500 rpm, someone with a good leg might still be happy that it "works satisfactorily." However, wisdom may come with age, but so do weaker legs. </p><p></p><p>Most of us, myself included, don't have tachometers attached to our kickstart levers that record the highest speed achieved when we jump on that lever. Which means most of us have no way of knowing if we have to kick them at 300 rpm, or at 500 rpm, or even higher to get them to start. However, I know that removing and replacing the armature significantly reduces the magnetization, and I also know my re-magnetized magnetos require a bit less than 300 rpm because I test them after re-magnetizing. I also know that bikes that start at 300 rpm are joys to have. Less so if 500 rpm.</p></blockquote><p></p>
[QUOTE="Magnetoman, post: 138643, member: 2806"] The issue with remagnetizing is all about how hard you will have to kick the bike to start it. Even an extremely weak magnet will generate sufficient voltage for a spark, [I]if[/I] the magneto spins fast enough, but that will be well past how fast someone can possibly kick it with an older "steel magnet" magneto from the pre-1930s. Alnico is more forgiving, but the effect is still present. . A Lucas manual says 300 rpm is the low end of kick starting speeds, with 500 rpm normal. At these speeds the generated voltage will be essentially linear in speed, so if a magneto has lost half its magnetization it will require spinning it twice as fast. What this means in practical terms is that if you had a magneto that supplied sufficient voltage for starting at 300 rpm, but then remove and replace the armature for any reason resulting in it now starting at 500 rpm, someone with a good leg might still be happy that it "works satisfactorily." However, wisdom may come with age, but so do weaker legs. Most of us, myself included, don't have tachometers attached to our kickstart levers that record the highest speed achieved when we jump on that lever. Which means most of us have no way of knowing if we have to kick them at 300 rpm, or at 500 rpm, or even higher to get them to start. However, I know that removing and replacing the armature significantly reduces the magnetization, and I also know my re-magnetized magnetos require a bit less than 300 rpm because I test them after re-magnetizing. I also know that bikes that start at 300 rpm are joys to have. Less so if 500 rpm. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Lucas K1F magneto weak spark
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