A couple of questions:
How many miles on the engine?
Have you watched the carburetor slide, or barrel, when you twist and release the throttle? Does it snap back down to the idle position or move down slowly coinciding with the engine rpm? Check your throttle cable for free movement. No kinks or binds in the cable. Lubricating the cable as well as the thimble in the twist grip where the cable wraps is always a good idea too.
A brand new engine will not have the compression that one that is broken in has and won't be as 'snappy'. You might have a very small air (vacuum) leak in the intake/induction system but I suspect a throttle cable/carb slide issue.
Tom