Hi, To me it sounds like your sync might be out. some Rotax people sync at 2500, some recommend and swear by doing it at 3000-3500 rpm. I have tried both and around 3000 works best for my setup .

I recall my first syncs didn't work out and I have had the fuel exiting the carb overflow. retried the sync , with some trial and error and experience syncing is easy and definitely smooths idle and lower rpms out.