The video from the factory shows the problem "very well" too;
https://www.youtube.com/watch?v=NnU7DUYLCII
It is a pity that they cannot bother to make a proper carb, it would have been very easy for them to incorporate a low end adjustment in their design.
The best for you might simply be too fit another carb, e.g. Enya or ASP (if you want a twin needle carb).
In order to get a leaner mid range and more gradual change than the air-bleed that suddenly opens, you can also change the diameter of the carb above the throttle barrel. I the bore is made slightly larger above the barrel it will effectively close after the opening below the barrel, and hence reduce the fuel draw at lower revs (while at full throttle it will not do anything).