That sounds odd. My throttle action feels as if it hits hard stops in both directions. You can check this by looking at the carbs while you twist the throttle. The operating arm on the carb hits a mechanical stop when the twist grip is fully open.
There are mechanical stops in the twist grip too where the top cover fits into a slot in the throttle tube.
I don't know a lot of detail about this but it's certain that all airhead throttle assemblies are not the same. There are at least differences
in the profile of the cam that the tiny chain rides on which will affect the relation ship between twist grip rotation and cable pull. I wonder if you have the correct throttle assembly and gears fitted.
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