Ok, I think I know what you are asking(if I'm wrong I apologize).
When you used your NT carb you got to 55kph but with the RT only 43kph.
If that is the case you may be 4 stroking(motor not firing every rotation because of a rich mixture) or just running too rich in the upper rpms with out any signs of four stroking.
Like I said in my other post to your other similar question, You need to do a plug chop to determine how rich you are running. You want your spark plug electrode strap to be a nice medium chocolate brown color. If its black and or oily you are running too rich and need to use a smaller jet. If its light grey/white you are running too lean and need to go up in jet size. Sounds like its running too rich and need to go down in jet size. Most stock carbs seem to be a little rich from the factory.
Hope this helps. Pictures of your spark plug would help us determine whats going on better.
