lack of power at high revs

[rigsby]

I have been racing an nc23 for a couple of years now, it has been super reliable and has required very little maintenance other than a new clutch. My last outing was at tonfanau where mid way through the second race it began to struggle anything past 12000 rpm. It was behaving like a diesel where it would pull well low to mid range then semmed to refuse to rev after 12000(it doesnt misfire or run rough jusnt wont rev). It hasnt had the carbs stripped since owning or plugs replaced. Does this sound like ignition or fuelling? some ideas of where to start would be helpful

cheers rigsby