I picked up a copy of Des Hammill's "
The SU Carburettor High-Performance Manual" to see what extra information I could get on how to get more flow though the pair of HS2's on the Spit.
The way I see it this is a point of restriction in the system, each intake port is nearly 1.25" in diameter all the way through the manifold up to the point where they merge at the carb outlet. Up until this point there is little restriction. To help this out I want to increase the flow capacity of the carbs to allow the pressure at the valve to be as close to atmospheric as possible. Then the only limitation on the engine is what the valve can flow. Anyways, here is a comparison between my prototype modified carb and a stock one.
I also found the book extremely helpful in initial needle choices and how to modify them reliably. There is a lot of information on this on the net but I found it a struggle to sort through. I might scan in some pages and put them in the manuals section. Ssssssshh!
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| Stock Butterfly, Shaft and Piston travel. |
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| Showing the stock piston travel better, all stock components |
I modified the spindle in two ways, I thinned the tapped side removing ~.050" and cut off the other side leaving ~.060 on either side to help retain the butterfly. I also cut a pair of tappers in the butterfly's holes to allow the screws to sit more flush with the face of the butterfly. When permanently installed I plan to flash solder the screws and butterflies in place for extra security. The increase in effective area at the butterfly is visually amazing. Flow simulations are showing a 20.6% increase in flow from 108cfm to 130.24cfm. With these two simple modifications. The springs were also shortened(1.25") to give a spring rate of 1oz preload as per Hammill's book.
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| Modified Spindle and Piston Travel (increased .100" as per Hammill) |
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| Better Showing the increased piston travel with modified spindle |
You can really see the piston is now completely flush with the through flow section of the carburettor and the the spindle takes up far less cross sectional volume than the stock version. Not bad for an hour of work and zero cost. The final version will be done on a pair of new spindles.
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| Piston Travel a bit more in focus, modified spindle |
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| Springs were shortened to 3.625" |
