Hi ppea
Congratulations on your new Grillo tractor!
My BCS 740 also has the Honda GX390 engine, and it has run without any problems for more than 230 hours since I got the tractor in the summer of 2011. I am therefore very surprised to read that you have had problems with your engine since day one.
I don't fully understand why you would think that a clone carburettor would be better than the original one? Considering the large amount of Honda GX-Series engines powering all sorts of equipment around the globe, the original carburettor seems to work well for most users.
As your engine is probably as new as your tractor, I would expect this to be a warranty issue? Have you had a talk with your Grillo sales/service person?
To diagnose your engine via this forum is of course difficult, but the fact that the engine surge at high speed and load, that the engine only stars when it is choked even when warm, and that a larger jet makes it run perfectly, all indicates to me that the engine is running too lean. Could it be, that an incompetent person at your Grillo store has adjusted the standard settings, and gotten it all wrong before delivering the tractor to you?
I hope you are able to get this issue fixed quickly, so you can begin to enjoy your new tractor.
Best regards
Jens