It sounds as if you starving for fuel at the upper end of the RPM range, open the HSN up some more until it will take throttle W/O any choke.
Then adjust the bottom as necessary for a smooth transition.
If that doesn't work there is something wrong inside the carb like a dirty screen or miss adjusted fuel metering needle. or even the gaskets on the pump side reversed. Try the easy stuff first(HSN) then if you need to dig into the more complicated parts of the carb.