The collision was reported to have taken place at around 5:30 p.m., according to the California Highway Patrol.
The CHP reported than an off-duty firefighter found the male rider on the side of the road, conscious but injured.
It was not immediately clear what caused the collision, which appeared not to have involved another vehicle.
South Lake County Fire Protection and Cal Fire, CHP and the Lake County Sheriff's Office were reported to have responded to the crash scene.
REACH air ambulance flew the injured man out of county to medical care, according to radio reports from the scene.
The man's name and where he was taken for care were not available Sunday evening.
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