Yamaha XZ550RJ 1982 Owner's Manual page 41

Fuel cock
The negative pressure fuel cock supplies fuel
from the tank to the carburetors and also
filters the fuel. The fuel cock has the follow-
ing three positions:
ON: With the lever in this position fuel flows
if the engine is running but stops if the
engine is not running .
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RES: This indicates "RESERVE". If you run
out of fuel while riding, move the lever
to "PRI" and then the switch to this
position after starting _ the engine. Fill
the tank at the first opportunity.