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RE: New engine for Scruff Part 2
Posted by : Neil (92.28.205.62) 
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On Saturday 01/10/16 at 19:57:09
Hi Harry. You are so right, carburation is the bane of my life. As soon as you up the size of the engine and carburettor the problems start. The standard Cub air boxes really play havoc, especially at wide open throttle at high revs, when the restriction results in less air flow but increased vacuum which pulls more fuel through the main jet, and so as you say, you end up running rich, even though the mixture was correct at wide open throttle at lower revs. One answer is to remove the air box, but keep the filter, but this is just as noisy as fitting an aftermarket cone filter, and the cone filters are less restrictive and once jetted correctly, provide better performance. Currently I am happy to sacrifice a bit of performance in order to make Scruff quieter for Cheri and I. The current set-up is a 25mm Cub90 style manifold, a Jingke carb with 25mm outlet (probably a about 22mm), a standard C90ZZ air box and filter, inlet pipe is a car radiator pipe sized to match carb and frame stubs. I have many different needles and jets to try and am slowly tuning it in for the best compromise. Incidentally the YX140 head also has a 25mm inlet port and I am running the standard cam. I’ll keep you posted. Where did you manage to bet your BBR manifold from and what size is it? I had to order my 25mm Cub90 style manifold from America....not cheap. The carb I use shares the internal dimensions as a Mikuni VM22, but has the advantages of having the air screw in the original position, it can use a standard Cub choke set-up, and the Cub style round float chamber means it’s overall height fits a standard manifold and clears the frame. There are plenty of Jingke carbs out there, but I have never seen another like Scruffs.
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