I have been running my bike on prime all spring with no "real" problem because  I was having a petcock/vacuum problem after cleaning the carbs for the second time over the winter (tank rust issue).  I purchased a new petcock but the bike still does not like to run in the "on" position.  Hoses are connected in the proper order and I added a clear in-line fuel filter to try and make certain the fuel supply remains clean.

My 500 is a 1990 with about 25k miles on it. 

I have searched most of the postings on this issue they were helpful.  Has anyone still had the vacuum problem after replacing the petcock?  Would appreciated any suggestions!  Thanks.

My only deal with prime is, its always on. The vacum will at least shut off the flow in the on position. Did you check both ends of the vac line?

I should have mentioned I did change all of the fuel lines when I did the last carb job...

But did you change the "suction" or vacumn line that goes from the caRB TO THE PETCOCK?

The last few weeks I've been making sure that my tankbag does not completely cover the gas cap and so far no fuel starvation.  (tankbags have been known to cause vacuum lock in tank when covering gas cap)

This may or may not not help you, but knock on wood, I think may have solved my fuel starvation problems

I've changed everything,and my petcock want operate at all in on mode.I keep it on prime.My petcock is the only thing I haven't changed.Any Idees?Anyone?Tanks.Ben

Raining here today but will change the vacuum line next weekend... if that doesn't work, gonna have to stick with "prime".  Thanks for the help.