Yamaha XZ550RJ 1982 Owner's Manual page 43

NOTE: _ _ _ _ _ _ _ _ _ _ _ _ _
In the "ON" and "RES" the cock works on
vacuum from the engine turning over. If the
line connecting the cock to the carburetor in-
take manifold is not connected or has a leak,
the cock will not function properly.
-31-
PRI: This indicates "PRIME". With the fuel
cock in this position fuel flows whether
the engine is running or not. If the fuel
tank is completely empty, refill the tank
and prime the carburetor in this posi-
tion and then switch to the
"ON"
after
starting the engine.