Motorcycle Repair: wont stay runnin, vent valve, fuel cap

Question
1982 yamaha virago will run for about twenty minutes the wants to die won't sta idelin

Answer
Sounds like you're running out of fuel.  When it dies, and you pull the fuel line from the carb, does fuel come out?  If not, I'm willing to bet that if you pull the fuel cap you hear a hissing sound and fuel starts to flow again.  Your fuel cap vent valve may be blocked.  Good luck,  Jan