I have a problem changing the deck belt on a troy -bilt ridding lawn mower model n779 pony. There is a guard on the deck idler pulley that will not allow me to put the belt on on the pulley.what would be the solution,what am I doing wrong?

You can remove the spring to the idler pulley arm, and you will have plenty of slack then to work with. To remove the spring, and to install the spring, use a stout piece of rope, such as pushmower pull rope, and hook on to one end of the spring and fasten the other end to a piece of broom handle to grip. Then you can stretch the fire out of a spring easily and without hurting yourself, but do be cautious and I would suggest safety glasses.