Its sounds as if its leaning out after the choke is open. Is it running long enough that the choke would normally be open? (engine warmed up?) Make sure there is no fuel restriction. (good flow to carb). Check to see if the bolts to the carb may have loosened causing a vacuum leak causing it to run lean. Open the low speed to a turn and a half and start from there. If the carb is tight and it starts to run good start leaning it out about an eighth turn at a time until it runs how you like on low end. I know on mine the low end will lean out to a point and then not want to start. If you have a fuel filter make sure it is clear. I know the engine is new but if all this fails take the cover off and look at the screen make sure no trash is in it.