dong,
My 66cc China Girl does the exact same thing. With the main jet soldered and drilled to provide excellent high speed running, and the needle set to the highest clip position (leanest setting), the engine runs too rich at low speeds just like yours, at up to around 1/2 throttle the acceleration is fine but once the bike reaches a stead-state speed, the engine begins four-stroking (firing every second revolution, an indication of excessive fuel).
This seems to be what happens with the NT style carb when you have the intake pipe and carburetor joints properly sealed, with no air leaks diluting the intake charge at high intake vacuum (small throttle openings). The bike will 'buck' and the engine will run rough, with excessive fuel consumption.
The only cure I can see is adding an air bleed, a small tube with an adjustable screw valve on it that bypasses fresh air from the air filter mounting face at the carb's air intake side directly to the outlet side after the throttle/jet tube. This way, at high intake vacuum, the air bleed can be adjusted to give the right amount of air bypass preventing excessive amounts of fuel from being drawn into the engine.
Almost every good small carb has an idle and slow-running mixture adjustment, either an air bleed like I described or a separate idle jet. These NT style carbs don't have that, and adjusting the jet needle clip position doesn't lean the mixture enough to provide smooth running at low speed.
I'll have to get around to installing an adjustable air bleed on mine... I'm too cheap to buy a different carb