You mentioned new carb - is it factory, rebuild, eBay bsr42 knockoff? Sounds like the engine is running way too lean, either starved of fuel or air leak in the intake piping. When the engine is running does it cough or pop?
My first advise would be to start at the intake piping. Check intake piping for air leaks. Remove the silicone carb boot (pipe between the carb and engine) and inspect for cracks. Then make sure the carb boot clamps are screwing in good and tight, sometimes these strip out over time and won't tighten enough. Critical to not have an air leak here... If all is tight, and still not running good, then move on to the carb. Remove the bowl, check jet sizes, clean pilot circuit, and make sure the pilot/mixture screw setting is correct.
If it's a new carb from a random ebayer selling hundreds of knockoff bsr42 carbs, id go straight to checking jet sizes.