Rule out fuel pressure first, ensure nothing has built a nest or plugged up the vent line to tank via applying a minimal air pressure to tank via fill cap. If pressure begins to build. Any, 2psi, stop. Your vent could be plugged, if so, clean the line. If these get plugged although you're sucking some, it may not be enough under proper cranking speed, and the rest would be vapor lock.. you won't be able to suck fuel past the pump pending where the piston is on the travel(assuming mechanical). If the lobes were worn on the cam, I would be concerned for the rest of the cam..
Rule out one thing first.
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