I have a similar setup. As stated there are 2 check valves in the fuel pump, one to keep fuel from back flowing from the carb to pump and the other to keep fuel from back flowing from pump to tank. They also work together to pump fuel under pressure to the carb but that's another story.
Another quick check you NEED to do is to check your engine oil level to make sure it hasn't risen or have a gas smell to it.
If the internal pump diaphragm leaks, gas can make it's way into the crankcase.
Hopefully that's not the case and the fuel is just back flowing between the tank and pump.
Good luck
Dave
