The fix John suggested will solve your problem. The Racor LG50 does two things. First, it prevents fuel from escaping out the overflow line. The second thing is does is increase the back pressure in the fuel tank, which is how auto pumps sense when to shut off. Becuase of the check valve in the LG50, the easiest escape path for fumes is out the filler nozzle. As the tank gets full, the pressure gradient gets higher which turns off the auto pump.
Before installing the LG50 on my boat I always had to play with the pump, turning it upside down and stuffing towels down the fill nozzle. Nothing ever worked and even though I always filled at the slowest level, fuel still ended up getting everywhere. Now I can fill at full throttle just like my car, and when the pump shuts off the tank is completely full.
Check out this article
for more info on the install and benefits.