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I've seen this with SU's on Triumphs. What is happening is when the engine reaches a certain rpm, it causes a vibration that causes the floats to rapidly "bounce" in the float bowl. Everytime the float bounces, a small amount of fuel is allow to enter the fuel bowl, even though the fuel bowl is already full, causing the fuel bowl to overfill and overflow (that why you're seeing fuel from the overflow tubes). This is especially true if you have intalled grose ?(sp) jets in the carbs.
A friend of mine autocross a '69 Triumph Spitefire with dual SU's and we found this was the case. We swapped the grose jets for conventional needle and seat configuration and the problem was solved.
While on this subject, I need some SU HS6 carbs with 1.75 inch bore. They would have come on a B18D engine. If someone has a set they'd be willing to part with please let me know.
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