First fuel pump I installed, would not pump. I finally took it off to manually work the arm. Putting a rubber hose on the Inlet side of the pump and working the arm I would get bubbles. Like it was pumping air. So i placed the rubber hose on the outlet side and it picked up fuel and pumped it out the "Inlet side". First I have ever seen, but that's what it did.

I already had another pump coming so traded it out.

Doing the same test I could get fuel out of the pump using the "Inlet side" and it would pump out the "Outlet". That was what I wanted to see with the first one but didn't. But on the engine it will not move any fuel even though gravity is providing fuel to the pump.

So with the first pump I think the valves inside were installed wrong. With the second pump it works off the truck if you manually operate the lever. But does not work when installed on the engine even with the fuel dribbling out the inlet line when disconnected. It's getting fuel, not pumping it while on the engine.

This why I think there is a problem with the cam lobe.

Right now I have a low volume electric pump on it pulling through the same line and it works fine.

Seen a lot of stuff over the years, but never seen a flat fuel pump lobe on the camshaft on one of these old sixes, but I can't seem to find any other explanation.