The oem spring looks quite compressed already in the photo, so you may be getting coil bind at full throttle. Perhaps shorten the spring?
To get a more progressive smoother throttle you can add length to the carb linkage so it travels through a larger arc.
The recommendation for dual springs for track use is a safety initiative, but here you have 3 springs, the two additional are overkill IMO, I would remove one.
Julian
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