5/16 to 3/8 fuel line

I`ve read that the "M code" cars came with the 3/8" pick-up and lines, but I haven`t owned one so couldn`t say with any authority. I believe the 5/16 line is adequate for cars deep into the 11`s. Like others have mentioned ,you may want to check the filter at the end of the sending unit and even the tubing itself for debris. If the car sat for any time then it`s possible the tubing is restricted with crud. Flushing with laquer thinner and pipe cleaners on a cordless drill will do a great job of cleaning the tubing on the sending unit.