hi it sounds to me like you need to check the air bleed screw!!!,back it out a little then put a small sowing needle or pin in the hole at the front of the carb and slowly tighten the air bleed screw till it just nips the pin but you can still remove and insert the pin but you can feel it niping the pin a little.if its a new engine run it a little rich for a cpl of flights it should be ok.but it sounds like the airbleed screw to me!!!. hope this helps.