how do I adjust the idler pulley on my weedeater lawn mower?

There really is no adjustment but I have had the same problem and it was either a cracked mandrel housing mount or the deck sway bar assembly bracket (part no 130017) that mounts on the chassis. The sway bar bracket had a broken bolt hole and was allowing the deck to move to one side thus reducing the tension on the belt. The mandrel housing mount which has three bolts would tip to one side when the blade drive was engaged. Both of these are relatively easy to repair. The bracket is under $10 or if you replace just the mandrel housing and reuse your shaft and bearings the housing costs under $20. These usually break when you hit an immovable object such as a rock. Hope this helps.

RWF
December 2007