My scooter is using a lot of petrol, it goes really slow to 25mph splutters and when it eventually gets to 30mph all of a sudden it has a random burst of power and goes up to 40mph very quickly it then splutters again and it is like it turns itself off and then fires backup again, i have been told it is lack of petrol, but why would it be using somuch petrol if that was the case? , is it running rich or lean? do i need to change the jet or adjust anything? thanks Mitch