Long subject
This could stretch into a long string of what if's. Fuel supply on the up stream side of the engine driven pump can be effected by a number of things. Line size, inlet blockage, sharp bens in the lines, fittings being drilled to small in the "I.D.". You name it. The best think to do is start at the pick-up and go down stream all the way step by step and check it all to be open and flowing as much as needed. The book by F.S.D.O. I think says 150% of what it the engine will need when it is at 100% power. I like to see a little more, but that's just me. It is not all that unusual to see some drop in pressure after the engine driven diaphragm pump in a fuel system of a low-wing aircraft. That is why we see a small transfer pump between the tank and the engine driven pump. That's the one you turn on at take-off and landing to be sure you get fuel to the engine driven pump out of a low wing aircraft, were the delivery pressure to the engine is most often negative and the engine driven pump must suck it up to it's location in the system. You get the idea, that is why I like wide open good sized lines with a little more than the 150% asked for. Fuel starvation is one of the top reasons for forced landing in low wing aircraft and high-wing. Just a thought here, Yours as always, R.E.A. III #80888