I run filters before and after the pump with no problems. if you run one before the pump it needs to be a high flow filter. I run a 10" stainless filter from
speedway .if you run a standard universal paper filter before the pump it doesn't take much to stop or severely restrict the flow from a mechanical fuel pump. I would also check your fuel pressure, my autozone mechanical pump was putting out 10psi so I had to run a regulator with it