The stock governor spring on the KX85/100 is too stiff causing the valve to open too late, which results in a dip in the horsepower curve. The Pro Circuit Governor Spring allows the valve to open at the right time, eliminating the dip in the horsepower curve and resulting in improved bottom end. Pro Circuit's Billet Stainless Power Valve combined with the Pro Circuit Governor Spring provide a major improvement over the stock set up