I can't stress enough... (as usual I know), that there is little use asking what other people are running as each bike can be considerably different. For comparison, when my 300 was running standard 21 carbs (now on PJ38's) I was running a full HRC jet kit with 'B' needles on position 4 from top and 175/172 mains, with 45 slow jets. (But this is with a premix set up) If you ask the sizes for other members with 300's, there is quite a range...

This was set up on the rich side also. (6.5% CO2 reading)

The best thing to do would be to start at 185 or 190, make sure it idles and revs once warm and run it in on a dyno, where thay can at least tell you you're safe rather than guessing. 70 quid for a session is better than a seizure!
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NSR300R - Why did i ever have a 250...