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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Air Fuel Gauge 02 Sensor Lambda Sensor
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<blockquote data-quote="Magnetoman" data-source="post: 107101" data-attributes="member: 2806"><p>Stop me if I've already mentioned this...</p><p></p><p>I spent some time last month messing with carburetors on my flow bench motivated by wanting to understand the effect the shape of the spray tube has on the pressure difference that pulls fuel into the airstream. It turns out an Amal Concentric intended for 2-strokes has a different shape spray tube, which is well known, but also a hidden restriction inside the compensating air passage that I've never seen written about before. However, hoping to make a very long story short, concentrate on the blue/red dashed curve and the purple 'pilot circuit' curve. </p><p></p><p>As can be seen, at 5 CFM (~1/8 throttle) the depression over the pilot jet is 3.06x greater than that over the needle jet. A #25 pilot jet has diameter 0.0175" for an area of 2.40x10-4 sq.in. The annular area of a 0.1065" needle jet with a 0.0984" needle is 13.0x10-4 sq.in., which is 5.4x greater. What this means is that at ~1/8 throttle, where Amal's simplified tuning description says the pilot jet is only just losing its influence, already the main circuit is supplying 5.4x/3.06x = ~1.75x more fuel than the pilot circuit.</p><p></p><p>At larger flows the depression in the pilot circuit saturates at ~0.150 psi whereas that of the spray tube of a 2-stroke body (dashed blue/red curve) continues increases to 0.340 psi at full throttle. Further, by that point the needle is essentially out of the needle jet and flow out the spray tube is determined by the main jet. Assuming a #300 main jet (dia. 0.059"; area =27.3 x10-4 sq.in) the relative flow of fuel through the spray tube will be greater than through the pilot circuit by (27.3 x10-4 / 2.40 x10-4) x (0.340 psi / 0.140 psi) = 27.7x. That's a pretty large factor, but it shows the pilot circuit is still responsible for nearly 4% of the fuel supply even at full throttle. For comparison, in the range where the taper matters, moving the needle by one slot in a Mikuni change the mixture by 4.5%, which indicates even at full throttle the ~4% effect of the pilot circuit isn't completely negligible. </p><p></p><p>[ATTACH=full]26992[/ATTACH]</p></blockquote><p></p>
[QUOTE="Magnetoman, post: 107101, member: 2806"] Stop me if I've already mentioned this... I spent some time last month messing with carburetors on my flow bench motivated by wanting to understand the effect the shape of the spray tube has on the pressure difference that pulls fuel into the airstream. It turns out an Amal Concentric intended for 2-strokes has a different shape spray tube, which is well known, but also a hidden restriction inside the compensating air passage that I've never seen written about before. However, hoping to make a very long story short, concentrate on the blue/red dashed curve and the purple 'pilot circuit' curve. As can be seen, at 5 CFM (~1/8 throttle) the depression over the pilot jet is 3.06x greater than that over the needle jet. A #25 pilot jet has diameter 0.0175" for an area of 2.40x10-4 sq.in. The annular area of a 0.1065" needle jet with a 0.0984" needle is 13.0x10-4 sq.in., which is 5.4x greater. What this means is that at ~1/8 throttle, where Amal's simplified tuning description says the pilot jet is only just losing its influence, already the main circuit is supplying 5.4x/3.06x = ~1.75x more fuel than the pilot circuit. At larger flows the depression in the pilot circuit saturates at ~0.150 psi whereas that of the spray tube of a 2-stroke body (dashed blue/red curve) continues increases to 0.340 psi at full throttle. Further, by that point the needle is essentially out of the needle jet and flow out the spray tube is determined by the main jet. Assuming a #300 main jet (dia. 0.059"; area =27.3 x10-4 sq.in) the relative flow of fuel through the spray tube will be greater than through the pilot circuit by (27.3 x10-4 / 2.40 x10-4) x (0.340 psi / 0.140 psi) = 27.7x. That's a pretty large factor, but it shows the pilot circuit is still responsible for nearly 4% of the fuel supply even at full throttle. For comparison, in the range where the taper matters, moving the needle by one slot in a Mikuni change the mixture by 4.5%, which indicates even at full throttle the ~4% effect of the pilot circuit isn't completely negligible. [ATTACH type="full"]26992[/ATTACH] [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Air Fuel Gauge 02 Sensor Lambda Sensor
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