I've just changed the carb rubbers on my NC30 & gave the carbs an overhaul while I was at it. I added extra 0.5mm shims to the needles also. Since putting it back together it sounds like its idling / running very rough with a lot of exhaust popping that wasn't there before. I've heard stories about the extra shims causing side affects. Is this what I'm experiencing?

Any help appreciated.

Did you balance the carbs at the same time? Popping and flames indicates an air leak so the carbs may not be seated correctly.

Yes, I balanced them while I was at it (drill bit method). The rubbers were extremely hard so maybe something didn't seat right but I checked the fronts with a torch & everything seemed ok?