The two small sprockets are worn out, and the nylon idler looks as if it is now eccentric, which would mean it's a goner as well. You should check your chains for pin and side-plate wear: hold the chain horizontally on its side and see how much it droops. The droop is due to slack in the pin joints, and it increases the pitch of the chain, which ruins the sprockets.
I don't recall a nylon idler on that mower - I think it should be a metal idler. The nylon thing may not have pivoted freely, and may have not been rotating, causing eccentric wear. You should fit the proper chain tensioning idler.
Last edited by grumpy; 13/07/11 04:18 AM. Reason: Add idler detail
