Snapper ZTR deck belt issues

[fedexpsd]

I just bought a used 2005 fast cut 50" deck that keeps throwning the deck belt. model zt20500bv serial 50236817. The previous owner had the bent deck replaced a year ago with a new one. I noticed that the three tensioner pullys were 4" pulleys and all of the other mowers that age that I looked at are 3 1/2" idler pulleys? So I replaced the 4" with the 3 1/2" and now the belt is too long, even with the tensioner bracket moved to the third hole? The part # on the new deck is 5049097D I think that the jack leg dealer (gas station) installed something wrong when he installed the new deck? I did notice that the new decks have two brackets between the two pulleys and mine and the other old ones only have one bracket on the bottom of the two idlers. Thanks for any ideas, Steve

[IRRITECH]

you can pull a parts manual from Snapper's website. Look at the setup of the deck and compare it to yours. Also check the pulleys on the spindles. I had the same issue on a 48" w/b last year and one of the pulleys was bent. Could not even tell bt looking at it.