Does your petcock say "on-res-pri" or "on-off-res"? If it is the second one, pull the line off at the carb and cover the exposed port with a vacuum cap. If it is the first one, replace it with a raptor version, and then pull off the line and cap the exposed port at the carb body...

The line can crack and fall off the petcock. Or the petcock was acting up and someone used the on and res spots as on off position and ran it only in pri. Or swapped to a manual petcock and forgot to cap the carb port.

Something is not right with your set up.  You have the stock petcock, and it is operated by the vacuum of the engine.  When the engine is running - the engine vacuum pulls the petcock open and allows fuel to flow.  The PRI position is to be used when you have either run out of fuel, or not run the bike for a while and the fuel in the float bowl has dried up.  The PRI position fills up the float bowl and makes it unnecessary to crank the engine with the starter to allow fuel to flow.

Does your bike run in the On or RES postitions?  It should not get any fuel flow with the vacuum line removed.  Perhaps the previous owner has modified the stock petcock.....and removed or disabled the diaphragm.