12mm sounds like the most common carb for the Big Cats (ST 2300 &up) which all sport a 12mm airflow bore. No first hand knowledge on this for the 2300 as I only have the 3250. But my experience with tank level was that there was not a single good high needle setting for both full and bottom of tank, even after sleeving the ST carb down to 10mm diameter bore. The fix for mine was a partial tank fill, only putting about 18-20 oz in a 24 oz tank. My 18-20 oz fuel level is just above the carb bore at that point.
The sleeved barrel creates fuel draw, but hi tank level floods it too much. My case is special in that I have no muffler back pressure because my Jtec muffler was 0 psi back pressure even with one pipe plugged.
That's why I went to the trouble of sleeving it.
I was also not interested in spending $45 on an engine that I bought from a friend "barely used" for $100 to go on a plane I paid $20 for.....
I have expensive projects and this is not one of them!

Page 5 at CLUB SUPER TIGER has a bit on this. Flyer95 talks about 9mm bore there. I am doing OK with 10mm after sleeving the carb throat down from 12mm. 9 would be easier to make, as the wall thickness on mine is just 1mm!