WOW! I just
submitted a post about the exact same problem. After two new pumps didn't fix it I added an electric inline fuel pump between the tank and mechanical pump as somebody suggested. That "fixed" it but not really, I wasn't buying it but maybe, like Otto experienced, both of my new fuel pumps were bad after all. I am now running a big block and wish I had measured the pressure going into the carb like Otto suggested.
