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We have a Cub Cadet 1045 LTX and the deck belt is very loose. After some googling, I don't see anything bent and it's a brand new belt (obviously, the right part). It was my dad who put it on so I suspect something didn't get attached the right way maybe, given his age, but I can't spot anything that would make it that loose. Any pointers appreciated!

Just to be sure, OEM# is 954-04219 or 754-04219
It should cone from the engine pulley, around 1 idler pulley, around the left spindle pulley, straight across to the next pulley and around one more idler pulley and back to the crankshaft pulley.

https://www.cubcadetpartsnmore.com/cub-cadet-service-manuals

see CUB CADET LAWN TRACTOR SERVICE MANUAL LTX