1991 Ford F-150 front fuel tank

I have a 1991 ford f 150 straight six. has dual tanks but when it runs on the back tank it runs like a champ but when u switch to the front tank it runs rough it stalls out and when you hit the gas it tries to die on you unless you have the gas peddle at a high running spot then it stays running. also the gas in the back some how goes to the front tank and pours out of the fuller neck if and where is the fuel tank selector switch

thanks Rusty