Half-throttle is primarily governed by a combination of the needle jet and the needle. Work with those to get past the hiccup.
125 main would be a good try for a main, because that is usually a decent jet for the Mikarb. If it needs a little more, then try the 127.5.
Usually, the problem jet on the Mikarb 28 is the needle jet being too lean, so a richer one is typically needed.
Unfortunately, that is the jet that is not compatible with any Mikuni jets, and it costs a lot more money to get one.
But, it is usually needed.