"Up"
on some bikes there's a screw blocking that direction. screw it out, and then you'll have 3 positions.
Up = PRI (Prime), can be indicated with a full circle.
Left= RES (Reserve)
Down= ON (Normal)
Normal is for normal driving duh.
Reserve you use when normal can't give you the juice anymore. In other words, start driving towards a gas station.
And Prime is used for refilling float bowls, fuel tubes, etc etc. It makes the maintenance job a bit easier.
The biggest difference bewteen on/res and prime is: Prime flows fuel no matter what. Like on dirt bikes. The other 2 settings are triggered with a vacuum. If you disconnect the tank you will notice no fuel will leak out, only if it's in Prime setting. When you start sucking on the other outlet (Please don't do this, let a tool do it for you
) then fuel will come out in setting On/Res too.
i suggest you download the service manual of your bike (B4, B6 or B12) from somewhere on this site, there are pics that might be more helpful in understanding it