I recently bought a troy bilt super bronco ,model number 13AT609G766 ,because my cub cadet tank zero turn was down. Well i found that it had alot of problems.the worst was that it was very slow .after weeks of working with it and studying manuels i found this mowers problem. I thought it was the belts were stretched but i found that the previous owner had taken it apart and put the springs back on wrong..The variable speed belt ,number754-0468 needs to be attached to the drive pedal assembly with two springs .numbered 732-0384 and 732-0963 . They attach to the idler bracket .number 683 -04130abd then drive oedal to give the belts enough tension. It nay not be your Broncos reason to go slower but i have read its a big problem for this tractor so i wanted to post my fix. Thanks .Howard