Motorcycle Questions? Ask a Mechanic.
Hi and welcome to the site, I’m Louie. Sorry you had to wait awhile but I’ll do my best to help.
Initially and depending on the version, the PRI position would allow fuel flow even if no vacuum (engine not running).
On the other hand and if no PRI position, then bypass would be to change the location of the spring pushing on the diaphragm. The diaphragm when there is no vacuum turns off fuel flow by the force of the spring. When there is vacuum, the diaphragm is drawn resulting to a valve opening allowing fuel flow.
The above of course would only be possible if the vacuum valve ass'y can be opened.