Gas comes through the small nipple on the bottom of the carb.
this happens when I shut off the bike or turn on the pet cock.
What would the cause of this be and what can I do to make it quit?
thank you.

Needle valve assembly worn or dirty. Or float is defective or out of adjustment. Romove carburetor take float bowl off and inspect and clean. You can still buy the needle valve assembly from Yamaha. See service manual here in tech library for float adjustment and specs and other info on the carburetor. 

Might be as simple as some rubbish stuck in the float valve--if you're lucky.

Thank you very much.