ok you will have a "genuine 30mm race Mikuni", known around these parts as a "Fakuni 26mm"....
They are shite, and should be replaced immediately... ha ha ha!!!
best thing to do with em, is remove them from the bike, immerse them in petrol, take 3 steps backwards, giggle, then throw a match at it. giggle some more.... pretty simple....
Failing that, you will need to start tinkering... this particular carby has a tendency to exhibit the symptoms you've described... sometime for a reason, sometimes they just up and shit themselves, with no fix available... just basically the carby failing...
You will need a #20 pilot jet, the needle clip on the middle setting, and a #92 or #95 mainjet......
there really is little choice but to whip the tools out and start tinkering mate.. carbies, particularly mini bike carbies arent rocket science in their construction... there are roughly 15 moving or removable parts to a fakuni carby, so grabs a phillips head, start pulling down, and you will see how they work pretty quick, it's a great way to learn....
As for the actual problem with your bike, it's just poor or inaccurate jetting... and is easily fixed with the right jetting combination...