Air Bubbles In Fuel Line

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Dart Into Traffic

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Hello. I need help from people more experienced than I at diagnosing fuel delivery problems.

Car = 1974 Dodge Dart Swinger factory 318.

Modifications = Summit Racing cam #6900, 2bbl to 4bbl conversion using factory Chrysler iron spreadbore (<--edited from squarebore) intake manifold, Quadrajet carb. NEW Carter fuel pump.

Symptoms: Runs fine until I stomp on it. From a stop it will go good, I can hear the infamous sound of the secondaries opening up (bbwwaaaahhHH!) Then after about 5 seconds it falls on its face. Almost stops running. Then something "catches" up and I can repeat the same procedure immediately and get the same result. I opened up the carb, readjusted the float to specifications, bench tested the float and level ok before reinstalling. I don't believe this is a "bog" issue of the secondaries as they weren't even opening when I first encountered this problem.

Test: Replaced fuel filter
Results: No improvement

Test: Ran with NO fuel filter
Results: No Improvement

Test: I wanted to see how "strong" the fuel flow was so I disconnected the fuel line at the carb. Ran a funnel and clear line to the carb to test run the engine with the fuel pump disconnected from the carb. Ran a second clear line from the output of the fuel pump into a small gas can. Started the engine. Bubbles GALORE. I tried revving the engine. More bubbles. I'm not talking one or two, I'm talking more than comes up from the bottom of an aquarium at the pet store. It did it both at idle and higher rpm.

Questions: Is this normal? Should it just be solid fuel coming through the line with no air in it? Am I sucking up air from somewhere behind the pump? Maybe with the rubber connections near the tank? I have NOT inspected the tank. Should I go get a fuel pressure gauge, a much easier task, before dropping the tank for inspection?

I really think the symptoms I'm experiencing come from fuel starvation. And although I know the Quadrajet has a small bowl, this wouldn't even make it half track down the strip. That can't be right.

Thanks for advice fuel delivery gurus
 
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Yes, it's sucking air probably from rubber line at tank. You don't need to drop it to check for that.
Good luck!
 
Did you say you have a Quadrajet on a Square bore intake???

That is going to be a major miss match. Big huge secondary butterflies trying to swing open in the smaller square bore openings.

Did you use a spread bore to square bore adapter?

Thinking that quadrajet is too much for your 318.

If you can get an Edelbrock 1406 600 cfm square bore carb, put that on there for a test > should straighten right out. They work very well for the 318s cammed or not.

☆☆☆☆☆
 
Yes, old fuel line at tank can crack and suck air, you should replace all rubber fuel lines with FI line it will last longer and with our special fuel here in CA you should plan on replacing it every year to be safe. If that doesn't help get a pressure gauge and if good you need to check volume of fuel GPM. good luck and have fun.
 
Check the hose at the tank for leaks as stated above if that don’t cure it your probably gonna need to check the sending unit pickup tube if it’s the original. There’s a chance there could be a pin hole rusted in it an easy way to check that is filler up and see if the air disappears.
 
Every line and filters from the sending unit to the carb needs to be checked.
Then pull the sender and check it as well.
If you have no air leaks in the line, then it almost has to be the fuel pump.
(Which by the way may be dumping fuel into the crank case if it can get air in the circuit)

Not trying to over complicate the issue, just solid diagnostics.
 
Just a thought...

Boiling fuel?

I think the pickup is most likely cause though.
 
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As others suggested, air is entering the line on the SUCTION side. So as well as a hose or fitting leaking, it could also be the new fuel pump.
 
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