Hi

Got a NC30 a few weeks ago. Cleaned carbs, new filters etc. Running 120 main jets up front and 122.5s for the rear cylinders. Both are new Mikuni jets I ordered (I know the bike have a Keihin Carb, but the supplier I bought from assured me they are the same). Standard pipe. Slide holes have been drilled (by previous owner) to 2.5mm. Pilot screws out 3 turns.

Bike goes really well up until about 11k rpm, but has a flat spot between 10.5k rpm and about 12k rpm. Then picks up and goes hard to 14k +. I have no idea what the problem is. I am thinking of changeing the main jets to 115 front and 118 for the rear. Should this help?

Any comments appreciated!!!

It sounds to me as If you have a mix of UK/Jap spec Carbs. Both my bikes were factory standard. The UK bike had 120/122 Jets in but metal slides with enough holes to not need drilling, whereas the Jap spec carbs had 116/118 Jets in and slides with the single hole in them.
I too have drilled the Vacuum holes out to 2.5mm in my jap carbs and now cannot get them to work very well at all. The bike is unrideable. Grrr.

It seems to me that you have got UK Sized Jets with Jap Slides, and I am not sure if they are compatible with each other.

Wait to see what other advice you get first, then if nothing else works i would try putting 116/118 jets in it and see how that is. mine did run perfectly on those settings, and still should. GRRR

I would get some Keihin jets for it. Mikuni jets sizings are not directly comparable with Keihin jet sizing etc.