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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Concentric Modification
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<blockquote data-quote="Nigel Spaxman" data-source="post: 68654" data-attributes="member: 2907"><p>I think that Norton Commandos are on the edge of suffering from insufficient flow from the Concentrics. I think that they compensate for this with main jets that are large. It is not the ideal situation. The standard size main jet for a Commando is about 260. My friend has a Commando 750 with a PW3 cam and I think he has 360 mains and it runs pretty lean even with the air cleaner. It seems that when he puts bigger jets in it makes hardly any difference. I have told him to modify his float bowls but he hasn't done it yet. Vincents would be in the same boat, but nothing pushes this system to the limit as much as two 414 cylinders sucking from one of these tiny float bowls. </p><p></p><p>For comparison a stock Triumph 65o Bonneville runs either 180 or 190 main jets, on a Trident the main jet standard size is 150. Niether of these engines pushes the limit of the 900 series or 600 series carburetors. I think it is when the main jet is nearly 250 that the float bowl and the float needle and seat become a problem. If the float needle restriction was not starting to cause a problem you would expect that a Norton 750 main jet would be more in the range of 190 X 750/650 = 219</p><p></p><p>You have to make sure you have the float needle seat with the big hole as well. I think the biggest one is either .125 or .156"</p></blockquote><p></p>
[QUOTE="Nigel Spaxman, post: 68654, member: 2907"] I think that Norton Commandos are on the edge of suffering from insufficient flow from the Concentrics. I think that they compensate for this with main jets that are large. It is not the ideal situation. The standard size main jet for a Commando is about 260. My friend has a Commando 750 with a PW3 cam and I think he has 360 mains and it runs pretty lean even with the air cleaner. It seems that when he puts bigger jets in it makes hardly any difference. I have told him to modify his float bowls but he hasn't done it yet. Vincents would be in the same boat, but nothing pushes this system to the limit as much as two 414 cylinders sucking from one of these tiny float bowls. For comparison a stock Triumph 65o Bonneville runs either 180 or 190 main jets, on a Trident the main jet standard size is 150. Niether of these engines pushes the limit of the 900 series or 600 series carburetors. I think it is when the main jet is nearly 250 that the float bowl and the float needle and seat become a problem. If the float needle restriction was not starting to cause a problem you would expect that a Norton 750 main jet would be more in the range of 190 X 750/650 = 219 You have to make sure you have the float needle seat with the big hole as well. I think the biggest one is either .125 or .156" [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Concentric Modification
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