Small Engines (Lawn Mowers, etc.): Drive Belt for Craftsman Lawnmower, transmission pulley, drive pulley

Question
I have a Craftsman 6.5hp 21" Mutli-Cut Front Propoelled Lawnmower model 917.378840.  The belt popped off the lawnmower that drives the front wheels.  I ordered the replacement belt and installed it on the lawnmower.  During the first cutting, the belt comes off again.  This time and last it got wedged between the engine idle pulley and the deck housing.  The spring that self-adults the drive pulley seems firm and working poperly.  There is a belt cover housing  that never seems to stay on.  There are no screws for the housing, just tabs to stick under the deck housing.  Any ideas on how to keep the belt from coming off?  


I link to the parts diagram can be found here..
http://www3.sears.com/Paul/partlist_search.asp?page_id=00002&prd_typ_id=1500600&...

Answer
You need to verify that the engine and transmission pulley are  aligned when the drive is engaged.  It usually happens that the issue is either with the incorrect belt, pulley alignment or something loose in the transmission mounting.  Hope this helps.