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Discussion Starter #1
Mower deck just started throwing the belt immediately on startup. Put it back and it immediately throws it again.

All spindles appear not be bound up and when the transmission is engaged with the belt off the pulley seems to spin true.
I don't see any obstructions below or atop the deck and the blades can be turned via the spindles. I am presuming that the three blades don't require synchronization to avoid each other.

I can't recall when the belt was replaced but I don't think it is original as it it looks ok. Maybe replaced once since 1997 but it's not cracked or frayed.

Deck has been stored outside since new in 1997 and is in original condition except for noted belt.

The idler tension spring doesn't seem to require significant muscle to remove and replace it so I wondered if that might be the issue.

Any ideas?
 

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Had same problem on my 420 50" deck. make sure all pulleys are level, I see you store it outside. On my 50" deck the idler pulleys had just enough flex to throw the belt off. I think has the decks detiorate the ilder pulleys mounts get rusty and allow the pulleys to flex. My main prroblem was my son wacked something and bent the pulley mount and when we straightened it back it just got weaker and weaker. I pulled the pulleys and took it to a welder and he reifnorced the pulley mounts and the lasst two years no problem. Good Luck only use john deere belts they have a different cut on them.
 

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Had same problem on my 420 50" deck. make sure all pulleys are level, I see you store it outside. On my 50" deck the idler pulleys had just enough flex to throw the belt off. I think has the decks detiorate the ilder pulleys mounts get rusty and allow the pulleys to flex. My main prroblem was my son wacked something and bent the pulley mount and when we straightened it back it just got weaker and weaker. I pulled the pulleys and took it to a welder and he reifnorced the pulley mounts and the lasst two years no problem. Good Luck only use john deere belts they have a different cut on them.
This is the place to start. When the deck first engages there is alot of wrap up in that belt until everything starts spinning together. Sometimes the tension arm will not pivot correctly to allow the spring to tension correctly.

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Discussion Starter #4 (Edited)
With the belt off the idler, it seems that the tension idler pulley bracket allows a small bit of vertical rocking at the bracket mounting bolt/shaft allowing the tension idler pulley to rise, maybe 1/8" or a bit more when rocked by hand. Not sure if this is normal or not. There is no play in the pulley mounting but the whole pulley can rock up that small amount due to the bracket attachment.
I'm hoping there are bushings on the bolt where the bracket is secured and perhaps they are worn or damaged.

The other fixed guide pulley seems to have a very small amount of vertical wiggle in the bearing/bushing but not nearly like the tension idler.

I hoped to not pull the deck out but that doesn't seem to be the case now.

On another note, I don't know how accurate website's parts descriptions are but my unloaded idler tension spring is a bit longer than the one listed as an oem replacement, both having the same number of coils. Maybe it's not as tight as it once may have been or maybe the manufactured length is not that fussy.
 
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