Sending it back was the best move.
The YS has a carby like no other. There are passages drilled in the carby itself that work very different to your normal venturi carby. I am guessing that one of these is blocked by maybe a sliver of swarf during manufacturing. Yours is way too touchy in the transition range. But one of our greatest problem fixer has always been the Enya #3 glow plug. Countless engine problems in tuning have been fixed with this plug. I don't think the plug would fix your issue, but it would be worth while having in your tool box for when the engine comes back.