THE NATURE OF A SERIES DC MOTOR/SERIES CART SYSTEM:  Series cart systems are very sensitive to changes in load.  The most noticeable time you would see this is on hilly terrain.  A series cart will slow down as you start to climb a hill and will slow down even more the steeper the grade.  When you begin to go downhill, the speed will increase, even to the point of being dangerous if the terrain is steep.  The driver should use the brakes to avoid motor overspeed as the motor will not limit the speed.  Unfortunately, these reactions are the nature of a series DC motor. While we might be able to make this slightly more or less sensitive, this nature will be present no matter what custom design we provide.  Be cautious when working with a series cart.  Never operate a series motor, especially at full voltage, with little to no load (wheels up).  If you want to make sure a series motor works after servicing the vehicle, depress the throttle only about 10% to avoid damage that would not be considered a warranty claim.