Hello, I have a 1979 F250 Explorer that I bought brand new. It has the 460 motor with Holley 4 Barrel, Duel tanks and has not been altered in any way.
When the truck is used daily or Bi daily it starts immediately and runs as it should. When left to sit for a few days or weeks w/o starting it, I am unable to start it due to a lack of fuel at the carburetor (as evidenced by nothing coming out of the accelerator pump when the throttle is opened.) At that point, disconnecting the fuel line from the tank(s) at the fuel pump (no fuel here either), connecting the pump to an outside fuel tank and cranking the engine results in a normal start. At that point replacing the fuel tank line to the fuel pump allows normal starts for as long as I choose to drive it daily but once it sits for a few days the problem returns. The Carburetor and Fuel Pump have both been replaced and the symptom has not changed.

That all said, My first thought was that I had a small air leak in a fuel line somewhere between the junction of the two tanks and the fuel pump so that would prevent drawing fuel from the tank after sitting. That might explain why there is no fuel in the line when I disconnect it from the fuel pump if it all siphoned back into the tank but it does not explain why the carburetor fuel bowls are empty. I also visually inspected and "sniffed" the lines for evidence of leaking fuel and found none. With that I'm out of ideas and would appreciate any comments that could point me in the right direction. Thanks in advance and have a great day

 

Probably two things here;
A small air leak could allow the fuel to run back, as you stated. I recently had to replace all fuel hoses on my diesel, no leak but it was sucking somewhere. With a carb, the air is vented at the fuel bowl, so no running symptoms appear like with a diesel. If your hoses are original, they should be replaced anyway. Check the diverter valve with a vacuum pump.
Secondly, corn oil gas evaporates quickly, make sure all heat shields are still in place, especially the thick gasket under the carb., if they left that off. or used a thin gasket a lot of heat is transferred into the carb, and it boils off during the cool down period.
Thankfully, you have a Holly, earlier 460s used an Autolite 4350, it's junk. I am swapping a 68 460 into my '67 bumpside, it will have a Holly when done.
Edit, your carb should have a removable plug for setting the float level, you could add fuel through that hole with an oil can or such, easier way to prime it than removing hoses.

 

Thank you for the suggestions especially the one regarding the better way to add fuel when carb is dry. Not familiar with the diverter valve but I have the factory manuals so I will do my homework on that . Hose replacement is going to be tough since it looks like I will have to drop the front tank to get at it but agree that all hoses should be changed. Just for the heck of it though I am going to try disconnecting the hose at the valve that switches the tanks and at the fuel pump and pull vacuum on it. Want to know if that will show anything even if I change the hoses. Thanks again and have a great night.

P.S. FYI When you outfit your 68 Motor with a Holley, I just got a rebuilt 4180 from National Carburetor In Jacksonville Fl and am very happy with it. They have older Holley rebuilds in stock or will rebuild yours if you wish to do that.

 

Sorry, should have said selector valve. Yes the tank will need to be dropped partially, but leave that till last and see if changing the others helps. And I can't overstate the value of the thick base gasket, it's expensive so people try to cut corners, but it really can make a difference.
I already have put a Holly on the Lincoln, for testing. works great. I also have a '79 F250 for parts, that will give me all the brackets and C6 transmission I will need, but it's still going to be a big job.