With engine off, and air cleaner removed work the throttle linkage at least halfway thru. You should see some gas squirt into the venturies from the accelerator pump. If not it's more than likely the problem.
An air leak could also cause this.. Check mating surfaces. Low fuel level in the bowl limiting the high speed jets could also cause starvation or what you are experiencing. Blocked / plugged main jets? Once it speeds up, how is the engine's power? So many questions but start with the accelerator pump then investigate further. The instructions that came with the carb should tell you how to set it up.. Just some thoughts for ya..:trink40:What type of intake, stock or aftermarket? How long has it been on there?
Sounds like a blockage or possibly accelerator pump needing adjusting. They are usually set okay right out of the box, but may need adjustment