I have a Yanmar 2gm20 F with just under 300 hours. My fan belt is fraying into a fine ,fluffy powder. I lost a belt in Vineyard Sound in August and the replacement is fraying in the same manner. I have tried tightening and loosening, both to no avail. Pulleys appear to be in line and no play in the sheaves.
I'm running out of ideas to try for what seems to be a rather simple problem.
If anyone else has had a similar problem and solved it I would love to read the solution.
Thanks to all who responded to this inquiry. I am going to be trying several of the suggestions and will post my results when I have them.