Likely more than one problem.
If fuel is continually flowing out the overflow drain, even with the petcock off:
a: petcock is leaking (see this post by Broken Veteran}:
b: float valve needle is not closing (because of wear, crud build up, or float is bad or maladjusted.
c: o-ring on the float valve seat is old, dried out, or worn, and fuel is leaking around it.