Is the linkage keeping the throttle from going "wide open" or do you think the brakes are just dragging, keeping it from reaching a speed fast enough to shift? If it's the linkage, look for an un-natural bend or something in the wire, cause at full throttle it shouldnt bind or apply the brakes at all.. If you think the brakes are just dragging, try loosing the 2 screws that hold the brake "pads" to the transmission mount.. Just loosen ecah screw a 1/4 turn or so and give it a go.. Be careful that your brakes may not work very well if you make a big adjustment..