Hi,

I've not much experience with modern machines but IIRC, you are
prevented from changing the tuning by much on these modern carbs so I
guess that eliminates that problem.

The hairline crack is interesting - It would be worth popping the carb
off and checking it out further as this may affect fuel flow. I would
definitely replace it - you will have to anyway, even if that isn't
your current problem, and see how you get on with that.

Keep us updated.

Alan.