Are you sure its a 38mm and not a 39mm?I only ask because if its a 39mm the pilot jet seems a tad lean and the 500,s are a pita to start when lean.Most run 58-62 I believe.Not sure on the 38,s.Also possible needle and seat although I would think the gas would be running out and you said it isn,t.Could try getting it running and as soon as you shut it off turn the petcock to off and see if it will start a couple times.

Keihins don,t use a needle jet so if the carb is worn badly it could pull fuel past the metering rod and cause a no idle condition.I'm someone else will chime in with some thoughts...