Einar,

Many carburettors, also from highly reputable companies, like OS and Rossi, have a tendency to run too rich in the mid-range, even if the idle and the high speed settings are perfect. The (wrong) taper on the idle needle is the culprit.

To somehow make the engine more tractable in the mid-range, it may be necessary to make the idle mixture setting leaner than you really want it. So much so that it may occasionally affect the idle to full power transition.

For Rossi there is a carburettor made by MVVS, that made this problem go away. For Irvine, I have not yet heard of a solution.

The problem is not a widely known one, so it must be minor.