Difference in fuel pumps?

This afternoon after much debate, I pulled the timing cover to check that by chance the eccentric was not in place. It was there, and looked fine. I have run the engine from the tank with an electric pump for at least 15 minutes, so the lines from the tank are fine. I am going to put it back together, and try a new pump, I am out of options at this point, this is not that complex a system.