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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Lucas KVF Rebuild - UK Recommendations?
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<blockquote data-quote="oexing" data-source="post: 179012" data-attributes="member: 1493"><p>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: </p><p> 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 ? </p><p> So this is what we want for kicking the beast at retarded, certainly to be safe from kickback ? </p><p> 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. </p><p></p><p>[ATTACH=full]62035[/ATTACH]</p><p></p><p>[ATTACH=full]62036[/ATTACH]</p><p> 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.</p><p> 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:</p><p></p><p>[ATTACH=full]62037[/ATTACH]</p><p></p><p>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. </p><p> 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. </p><p> 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. </p><p> 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.</p><p> Or am I wrong again ?? </p><p></p><p> Vic</p></blockquote><p></p>
[QUOTE="oexing, post: 179012, member: 1493"] 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. [ATTACH type="full"]62035[/ATTACH] [ATTACH type="full"]62036[/ATTACH] 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: [ATTACH type="full"]62037[/ATTACH] 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 [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Lucas KVF Rebuild - UK Recommendations?
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