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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Unsteady Idle
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<blockquote data-quote="greg brillus" data-source="post: 136538" data-attributes="member: 597"><p>Well a magneto does have a "Range" of travel per the spark output.......On the test bench you can see quite easily that moving the cam ring (advancing and retarding the spark) in the full advance position the spark will be strong and if you advance it a little more, the spark drops off entirely, just like an on/off switch. This is because the "Break point" at which the end of the armature segment has passed the magnetic pole at the top and bottom of the mag housing, at this point the magnetic field collapses. then immediately starts to build again but with the polarity reversed (a magneto is an AC device) as the armature turns and the next build up of electrical energy starts again........As you retard the spark the magneto will still give an output due to the stored energy available within the armature core, until the contacts open and this electromagnetism collapses thus roughly the 200 volts built up in the primary winding x the secondary windings at a ratio of about 1:60 gives a secondary output of about 12000 volts.........This range as you retard the magneto varies from one to another........As you know a lot of prewar mags had a range of about 20 degrees (40 at the crank) A couple of reasons the mags loose some energy from when they were new, is loss of magnetism, but another very important one, is the diameter of the armature is very critical........An armature that has been machined down by say 10 to 20 thou on its OD will definitely have a weaker spark especially at slower turning speeds........So this coupled to a loss in the magnetism will definitely make for hard starting especially on a big engine that is hard enough to kick over........this machining down of the armature core is very common and irreversible.</p></blockquote><p></p>
[QUOTE="greg brillus, post: 136538, member: 597"] Well a magneto does have a "Range" of travel per the spark output.......On the test bench you can see quite easily that moving the cam ring (advancing and retarding the spark) in the full advance position the spark will be strong and if you advance it a little more, the spark drops off entirely, just like an on/off switch. This is because the "Break point" at which the end of the armature segment has passed the magnetic pole at the top and bottom of the mag housing, at this point the magnetic field collapses. then immediately starts to build again but with the polarity reversed (a magneto is an AC device) as the armature turns and the next build up of electrical energy starts again........As you retard the spark the magneto will still give an output due to the stored energy available within the armature core, until the contacts open and this electromagnetism collapses thus roughly the 200 volts built up in the primary winding x the secondary windings at a ratio of about 1:60 gives a secondary output of about 12000 volts.........This range as you retard the magneto varies from one to another........As you know a lot of prewar mags had a range of about 20 degrees (40 at the crank) A couple of reasons the mags loose some energy from when they were new, is loss of magnetism, but another very important one, is the diameter of the armature is very critical........An armature that has been machined down by say 10 to 20 thou on its OD will definitely have a weaker spark especially at slower turning speeds........So this coupled to a loss in the magnetism will definitely make for hard starting especially on a big engine that is hard enough to kick over........this machining down of the armature core is very common and irreversible. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Unsteady Idle
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