Since you're bike has two tanks the left side cap is not vented. The vent is in the high side or right hand cap. You're cap was either loose or the gasket has gotten old and failed. Can you post the same pic with the cap off?
The caps are standard Harley style and can be purchased on line from aftermarket suppliers in many styles. You will need both a right and left hand thread cap.
If you are going to paint you're tanks be sure to check the liner inside the tank. They are known to fail also and should be replaced before re painting.
