PRI, on, res, where's the off?

[rritterson]

when you drain the tank and remove the carbs you don't have any fuel in the system. That's when you swtich it to 'prime'. On prime, fuel can just run straight into the carbs without the motor on, 'priming' the motor to start.

Also, if you live in a cold climate, when you store the bike for the winter, it's common to turn the tank petcock to off and then run the bike until it dies. That way, fuel doesn't sit in the carbs for months and clog them up. When you bring it out for the spring you have to use prime again to fill the carbs.