Fuel cap not venting properly or something else?

[BadMotorScooter-IAFF]

I am having an issue with the fuel tank on my bike. I believe the fuel cap is not venting properly creating a vacuum effect, which halts the flow of fuel into the carb. I have to unscrew the fuel cap to allow the fuel to flow again. anyone else having this issue? Fixes?

 

[shell shock]

search function is your friend. here is a freebie spoon feed:
http://motoredbikes.com/showthread.php?t=30474&highlight=fuel+cap+venting