So, my 82' XJ650 does this: Driving around in PRIME position on the petcock, it runs fine.
If I flip it to ON, it will eventually stall & won't start, unless I go back to PRIME.
So, in this case, will a petcock rebuild kit help?
Or do I need a new diaphragm thingy, or new petcock altogether?
What I did to try & help was to remove the petcock from the tank, & blew out the passageways.
Not sure how/if I can actually check that the thing is sucking fuel in ON position, since it needs vacuum from carbs while running, right?

thx fellas..
Rich

Did you check to see if the vacuum hose is clogged or cracked? How about the intake boot between the carb and cylinder?

I did verify that I'm getting good vacuum suction from the carb going up to the tank.(smaller line)
Intake boots were recently 'repaired'
That is to say, I gooped them with permatex to fill in cracks...
forgive the patch job, but no $$ at present for new boots...

As stated, it does seem to run fine in PRIME setting....

thx!
Rich