Yes, I've come across one dirt bike back in FO, not too far from where you mentioned. It was right at the Jack's Rd & trail 80 area. He came blazing off of 80, crossed Jack's Rd, and then headed of to Lookout Ridge. We were pretty disturbed by that and fortunately came across a ranger in a BLM truck just a few minutes after that encounter. We told them of the incident. Not much could be done at that moment. However, they're aware that motor bikes are out there on the trails ... illegally.
-Shelley
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Shelley Potts
http://shelleypotts.com