Loosen the two bolts that hold the throttle control housing and slide it slightly down the bar. I'd bet it's binding a bit.
All these bikes putt-putt when stock. The carbs a set very lean from the factory. If you still have the air injection installed (shiny tubes right in front of the spark plugs), you should remove them with an AI removal kit. You also need to adjust your air mixture screws and balance the carbs.
