Hi all,
I have a 1992 NC31 that just won't run properly in that it feels as if it's restricted in some way. I've stripped and rebuilt the engine so no problems there, I've also taken off the airbox fitted K&N's and re-jetted the carbs but still wouldn't pull the skin off a rice pudding. Any ideas will be much appreciated.

Have you checked for the restrictors in the carb mounting rubbers

What jets have you fitted? What's the condition & colour of the spark plugs?

There are no restrictors in the carb mounting rubbers and the plugs are all running as normal. I thought it might be the CDI unit but as all the plugs are a good colour I'm just not sure. I have heard that these bikes were restricted from high speeds by something in the speedo so may have a look at that.
I should say that the frame is an NC 31 but the engine is NC 23.

The engine revs cleanly and quickly in neutral? If not, does it run clean to 5000rpm and then develop a miss-fire, stumble. Does engine performance improve at >10000rpm. Modified or missing airbox can have detrimental effects on engine performance at low rpm. Speed re-strictor only becomes active at >130kph.