I had the same issue on my BT7. Found a reference document on-line somewhere dealing with SU carbs. In it, the author recommended loosening the clamps that tighten the springs on the throttle shafts and let the springs "relax" (unwind). Then rotate the springs and clamps until you feel the slack is gone. At that point, continue rotating another 1/2 turn. Do that to each shaft spring. There is one for each carb. That will put enough tension on the spring to close the carb butterfly but not enough to make it hard to open the throttle. I found this worked perfectly and made the accelerator pedal easier to modulate. Especially when I let someone else drive it.
Maybe someone remembers where to find the article on the SU's.
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Rob Glasgow