Fuel System Picking up Air

[Ben w]

Initially I did not know what was wrong with my fuel system, only that my truck was spitting and sputtering more and more frequently. I started out by rebuilding my Quadrajet Carburetor and replacing the fuel pump that already appeared to be new from the previous owner. I also replaced the fuel tank selector valve. At one point in the early summer when things first started to get particularly hot, I thought that maybe I was experiencing vapor lock. Based on that, I bypass the steel fuel line that runs inside of the frame and ran a rubber fuel line from the back of the truck to the engine compartment. When that did no good, I went back to the steel fuel line and also replace my carburetor with a new Edelbrock AVS2. While tuning the new carburetor I noticed that air was bubbling up into the fuel filter. I replaced the fuel selector switch a second time and changed to an electric fuel pump up in the engine bay. Today I moved the electric fuel pump to the back of the truck, just in front of the tank selector valve but air is still getting introduced into the line. Do I just need to replace the sending units? I have replaced every piece of rubber hose between the tanks and the carburetor as well. The sending units seem to be kind of a pain to get a hold of and frankly I really want my truck running this weekend. Is there something else that I'm missing? I would think that if the issue was a hole somewhere in the system, it would be leaking fuel now that the pump is at the back of the system. That's simply does not seem to be the case. Any thoughts would be appreciated.

[VileZambonie]

Have you tried removing the gas cap? Is the tank properly vented?

[prider5]

Sorry to reply on a post this old - I'm experiencing the same problem, did you ever figure out what your issue was?

[bd]

The OP hasn't been back on the forum since he posted the thread in August 2021. 

What year/model truck?  What engine?  Carbureted or injected?  Naturally aspirated or forced induction?  Have you tried Vile's suggestion yet?

[prider5]

Thanks for letting me know bd! I can make a new thread if you'd like.

Truck is 1977 K20, with a lot of parts from a 1980 K10. 350 SBC with a 1906 carb, and has dual tanks.

I've been rebuilding the truck, so it currently has 2 new fuel senders (dorman iirc), new tank switching valve, and just recently put a new mechanical fuel pump on (after having fuel delivery issues). I thought maybe the hard line had a hole somewhere, so I recently ran new rubber line from the switching valve up to the engine and made sure I ran it away from heat (thought it was potentially vapor locking), and back when I put the sending units in I ran new rubber lines before the valve as well.
I have a clear filter after the fuel pump before the carb, and I can see it pump fuel well when I first start it, and then after 10 minutes or so of driving the filter runs dry, with what looks like a lot of air bubbles flowing in as it runs out. Been struggling to figure out what the issue is, contemplating just putting an electric pump on it, but it would probably need to be after the switching valve, and I know they are notorious for not wanting to pull fuel and going out, so not sure. I think I tried it without the cap on as Vile suggested, but it's been a bit now and I'm struggling to remember for sure ha. I'll try this weekend if I can find the time to go and test.