A brand new engine or a worn out old engine tend to blow out little ground up metal bits that seems to end up going through the pressure line into the fuel tank. I have never seen it so bad that the engine goes dead stick from it but what the heck, I have seen some odd things over the years. There is also a baffel plate in most mufflers, if that is loose inside your going to see a lot of gunk in the pressure line depending on where the pressure nipple is located. If you can you could try to see if it's tight enough or just remove it. If it's a new engine then it will go away by about the first gallon of fuel used. If it's an old engine wearing out then it will just get worse with use.