Repairs drive belt keeps coming off

Hi everyone, I'm new to this forum and glad i found it. Hopefully someone has experienced this or can help me with an answer. I have a Craftsman LZ 1000 Briggs and Stratton 21 HP engine, 42 inch deck, model number 917.271841. I can cut from anywhere from 1 hour up to 10 minutes and it throws the drive belt at the engine pulley. I bought the drive belt from Sears parts direct, the numbers match with with whats in the parts book, the belt seems extremely loose! One question i have are the belt guides supposed to be close enough to keep the belt from coming off? As of now they are about a half inch away from the engine pulley. My other thought is could i have worn pulleys or something along that line. When the belt is on it pulls great even through very thick grass and i have no other problems beside the belt coming off and usually at the most inconvenient times!! Thanks for any help, suggestions!!

The 'keepers' should be no more than 1/4" from the belt. After fixing that, check to be sure the belt tension spring linkage if not bound up from being fully tensioned by debris. Some models also allow you to apply more tension by tightening an adjustment nut.