Just to clarify a touch,....there is no difference tween prime, run, or reserve, as far as the running of the bike.
Run = gas will flow from the tank only when the engine is being cranked and/or running, but will drain the tank only down to the reserve amount
Prime = gas will flow from tank and have gravity pressure to the carb all the time, but will drain the tank only down to the reserve amount ??
Reserve = gas will flow till the tank is completely empty as long as the engine is being cranked or running
ps: Did I say that right I hope ?
On edit,.....I stand corrected,
"prime" will drain the tank dry