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Old 05-02-2013, 07:05 AM   #11
Beastly Adventurer
Joined: May 2005
Oddometer: 2,348
Problem with tuning CV carbs with white markers on the twistgrip is that you have no way of knowing where the piston/slide/needle is, as the throttle is only connected to the butterfly.

The position of the slide is dependent on the vacuum above it, and the strength of the spring above it which slows down the vacuum induced movement.

The mixture in transition is not solely dependent on the needle/ jet relationship either - if the butterfly opens faster than the slide opens the flow is faster above the jet outlet, which draws up more fuel and richens the mixture. So the strength of the spring affects the mixture too.

The later piston springs have a different part number, and are noticeably stiffer than the early ones, so if you use the later jetting at least try the spring that is listed with the jetting - getting the spring right can make a big difference.
Adelaide Hills, Australia. 93 R100 GS, 77 R75/7 ,70 BSA B44VS, , 86 R80 G/S PD, 95 BMW Funduro F650 ST
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