Never thought this day would come

[schnowz]

Have you tried running it just on prime?
That'll tell you if its a petcock or vacum issue to the petcock. Sometimes the vacum tube can get kinked and collapse. If you've tried different Gas caps already, then I doubt its that.
I'm guessing you have a clear fuel filter to tell you its fuel starvation and not electrical.
 
I had a slightly different problem . It was alright at low revs but as soon as I opened it up it would die - it turned out to be the rubber plug from the battery terminal got sucked into the air intake.
 
Pete S