I have a craftsman lawn tractor, model #917.272681. I had to change the mower blade drive belt and it seems really loose. Is there a way to tighten it?

Your deck should have a pulley for each blade and two more pulleys, one is stationary and just spins. The final pulley is your tension and is spring loaded. Make sure you put the spring on correctly after the replacement job and look for a threaded shaft that can be turned to apply more pressure.