Sounds like a petcock problem. Verify that your petcock knob is in the correct location which is pointed downward. Then, you need vacuum to activate the internal petcock valve. Look at the vacuum lines at the petcock to make sure they are attached and not cracked. Also look at the vacuum lines elsewhere in the system to see that they are not cracked or loose. If all of those things check out, draw a vacuum on the appropriate port of the petcock and see if fuel flows. If not, then the internal "guts" of the petcock need rebuilt.