Small Engines (Lawn Mowers, etc.): Craftsman lawnmower problem

Question
I have a Craftsman 6.75 horsepower 22" resr discharge self propelled lawn mower. The problem is that it doesn't self propel anymore. I changed the v belt but then I notices that the drive pulley that the v belt is on gets loose and comes off. I think this is the problem. How do I keep the drive pulley from getting loose when I start the engine. By the way, the I have no problem starting the lawnmower. It's just that i now have to push it myself.  The model no. is 917.377582.

Answer
From looking at the diagram of this unit on Sears.com, I would bet you have a broken spring that tensions the belt. I can't say I'm intimately familiar with this unit, but if you go to the Sears website and click on the parts central tab, enter your model number and you can see the same diagram I looked at.