OK here are my thoughts.....If I read this thread correctly, you get 5 lbs of pressure with the mechanical pump when cranking the engine, but you do not get 5 lbs when the engine is actually running and the electric pump is not in use. The engine does appear to run normally in that situation. Question 1 Did you run the engine rpms up and hold them there to see if the engine stumbled or was erratic after a period of time at the elevated rpm while using only the mechanical pump? Did CHT temps increase from a possible lean condition even if the engine wasn't erratic? Apparently the engine runs fine when the electric booster pump is employed. The thing that changes at that time is that you have fuel being forced under pressure greater than gravity, so it might overcome some type of blockage in the line or push the fuel thru the mechanical pump at the higher pressure. My conclusion is that the mechanical pump is probably OK because it does push 5 lbs when cranking. When cranking, less actual flow to the engine is needed than when actually running so even with a restricted flow the pump could show 5 lbs. When running more flow volume is needed, so if the flow getting to the mechanical pump is restricted ......the pump does not now have enough volume available to create 5lbs....but it could still be flowing just enough to run the engine. Using a container with a hose connected to the pump and providing gravity flow should tell you if the problem you are having is from the pump and after the pump . If you then have 5lbs of pressure the problem has to be in the components prior to the pump. If that happens then I would check your tank vent and blow some air through the fuel line from the tank. Don't really know if thats whats happening, but from what I understand about the situation, thats my best guess.