Sounds like stale fuel. I would drain it and start with some fresh fuel. Disconnect the main fuel line at the carb and drain the tank. As for the carb, there is the main jet cover on the underside of the carb that looks like a large bolt head but, you do not have to remove it. Instead, when you drain it, there is a flat blade screw valve that will drain the fuel bowl out through a black fuel line that is routed from the bottom of the carb to the gap between the rear engine case and front of the swingarm. Just by loosening the screw valve will allow the fuel to be drained so there is no need to unscrew all the way out. Try new fuel and see what happens.