I just completed a rebuild; never did this before. I had it running (badly). Put an aftermarket oil pressure guage, and it hasn't run since. It will crank and start very briefly, then dies. It has spark. It has fuel pressure. I can inject gas manually in the manifold, it will run briefly at a high idle. It acts like it's not getting enough fuel. I've checked the relays, and they are working. I'm baffled, and I need help. Has anyone experienced something like this?