Be sure on new motors to keep the carb vertical on assembly
This picture shows a bad mounting ... do not allow the the carb to be at a angle.

This movie identifies a rich running condition caused by the carburetor mounted at an angle or the main jet being too large for the conditions. If the rpm's decrease after half throttle is exceeded it is a clear indication of the problem and the position of the carburetor should be checked as well as any potential obstructions in the air intake.The main jet size can be reduced from a 160 to a 155 is density altitude conditions warrant.

THIS video shows the CORRECT situation. Correct assembly and proper running.