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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet Timing & BTH Advance Curve
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<blockquote data-quote="BigEd" data-source="post: 51285" data-attributes="member: 161"><p>A few points that may be taken as <strong>my </strong>observations or views but <strong>not necessarily absolute fact</strong>.</p><ul> <li data-xf-list-type="ul">It is quite difficult (not impossible) to get accurate dynamic (i.e. engine actually running) ignition timing figures with a stroboscope on a Vincent.</li> <li data-xf-list-type="ul">Whatever the <u>theoretical</u> timing figures it is what happens when the engine is running that matters.<br /> </li> <li data-xf-list-type="ul">The graph posted is "an <strong>approximate</strong> graph for the later BT-H <strong>twin </strong>magnetos electronics".</li> <li data-xf-list-type="ul">The later BT-H <strong>twin </strong>magnetos electronics were "tweaked" a little so that it started to spark at a slightly lower rpm. A side effect was that this slightly extended the effective advance range. <br /> </li> <li data-xf-list-type="ul">The theoretical range of say, an electronically controlled advance curve may not be the same as the actual range. The actual advance range cannot start until the magneto until the magneto is turning fast enough to generate sufficient energy to initiate a spark. (A battery/coil/ and points system doesn't have a starting to spark threshold.) Most electronic systems will stop advancing at a fixed rpm and some, at high revs, may reduce advance by a degree or two. <br /> </li> <li data-xf-list-type="ul">The lines on a graph may be extrapolated at the ends so are not really relevant because of what was said in the previous point.</li> <li data-xf-list-type="ul">The ignition timing should be set at full advance position rather than an assumed fully retarded figure. If the actual advance range is greater than the theoretical you could end up with too much advance.</li> <li data-xf-list-type="ul">An example of this applied to a mechanical advance retard mechanism might be a worn unit or one that has had the "arms" adjusted to limit/extend the travel of the bob-weights. (I think this is one of the things that MartynG found when doggedly sorting problems on his Comet.)</li> <li data-xf-list-type="ul">My <u>Rapide</u> has the later BT-H electronic advance curve. The ignition is set, as accurately as I can measure, to full advance around 34° before TDC. It almost invariably (touch wood - fingers crossed) starts first or second kick and has performed well with this setting for 25-30,000 miles. We often look to theory and printed figures when making our initial settings From this point we can adjust to find what works for our own machine.</li> </ul><p>Discuss. That should keep this thread going for a while longer.<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink ;)" loading="lazy" data-shortname=";)" /></p></blockquote><p></p>
[QUOTE="BigEd, post: 51285, member: 161"] A few points that may be taken as [B]my [/B]observations or views but [B]not necessarily absolute fact[/B]. [LIST] [*]It is quite difficult (not impossible) to get accurate dynamic (i.e. engine actually running) ignition timing figures with a stroboscope on a Vincent. [*]Whatever the [U]theoretical[/U] timing figures it is what happens when the engine is running that matters. [*]The graph posted is "an [B]approximate[/B] graph for the later BT-H [B]twin [/B]magnetos electronics". [*]The later BT-H [B]twin [/B]magnetos electronics were "tweaked" a little so that it started to spark at a slightly lower rpm. A side effect was that this slightly extended the effective advance range. [*]The theoretical range of say, an electronically controlled advance curve may not be the same as the actual range. The actual advance range cannot start until the magneto until the magneto is turning fast enough to generate sufficient energy to initiate a spark. (A battery/coil/ and points system doesn't have a starting to spark threshold.) Most electronic systems will stop advancing at a fixed rpm and some, at high revs, may reduce advance by a degree or two. [*]The lines on a graph may be extrapolated at the ends so are not really relevant because of what was said in the previous point. [*]The ignition timing should be set at full advance position rather than an assumed fully retarded figure. If the actual advance range is greater than the theoretical you could end up with too much advance. [*]An example of this applied to a mechanical advance retard mechanism might be a worn unit or one that has had the "arms" adjusted to limit/extend the travel of the bob-weights. (I think this is one of the things that MartynG found when doggedly sorting problems on his Comet.) [*]My [U]Rapide[/U] has the later BT-H electronic advance curve. The ignition is set, as accurately as I can measure, to full advance around 34° before TDC. It almost invariably (touch wood - fingers crossed) starts first or second kick and has performed well with this setting for 25-30,000 miles. We often look to theory and printed figures when making our initial settings From this point we can adjust to find what works for our own machine. [/LIST] Discuss. That should keep this thread going for a while longer.;) [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet Timing & BTH Advance Curve
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