[wbnapier]

I have a '65 GTO with a dead end 5/16" gas sending unit and feed line.

My mechanical fuel pump has an outlet for a ¼" fuel return line.

Is there any issue with running a return line to the ¼" gas tank vent tube on the filler neck, and changing the cap to a vented cap?

I ask, as there doesn't seem to be a gas tank sending unit available with a 5/16" + ¼" return nipple.

The reason is tough starting due to boiling fuel in carbs after heat soak.

[Cardo]

A/C cars have the return nipple in the sender. That sender is available through our host, plus many others.

[wbnapier]

Our host only has the sending unit with ⅜" feed and ¼" return according to their catalog and my check online. If I am wrong, would you mind sending me the part number?

[Cardo]

Wow! You're right.... they must have stopped making them...? I'll look around here today, I believe i have a new one around here that came with my car that i won't be using. I put a 3/8 fuel line in when i did my car.

[geeteeohguy]

Running a vented cap on a '65 will let gas leak out on acceleration if you have more than 1/4 tank of fuel. I would use the later sender with the 1/4 outlet. That said, boiling in the carbs after shut down won't be cured by this. Modern fuels boil at 140 degrees and are not formulated for carbureted cars. Nature of the beast these days.

[wbnapier]

Thank you for that info. The vented cap is non-starter then.

I was hoping that with a return line and phenolic insulators, it would mitigate the issue substantially.

[ZnbOlds455]

Originally Posted by geeteeohguy

Running a vented cap on a '65 will let gas leak out on acceleration if you have more than 1/4 tank of fuel. I would use the later sender with the 1/4 outlet. That said, boiling in the carbs after shut down won't be cured by this. Modern fuels boil at 140 degrees and are not formulated for carbureted cars. Nature of the beast these days.

Are you referring to the filler neck or the gas cap itself?

[geeteeohguy]

Originally Posted by ZnbOlds455

Are you referring to the filler neck or the gas cap itself?

Vented gas cap will cause leakage on take-off.

[jeff chmura]

I used the 3/8" sender with the 1/4" return
I just used 3/8" hose from the sender to the 5/16" hardline.
The hardline is barbed and it will hold 3/8" rubber hose, just tightening
the clamp

[JSchmitz]

Originally Posted by geeteeohguy

Vented gas cap will cause leakage on take-off.

I have a '64 GTO that has always had a vented cap. Car used to run 1.6x 60' times. I've never seen any evidence of fuel coming out of the cap. Please explain your experience with this. I also rarely fill much more than a half tank.

[Singleton]

Originally Posted by JSchmitz

I have a '64 GTO that has always had a vented cap. Car used to run 1.6x 60' times. I've never seen any evidence of fuel coming out of the cap. Please explain your experience with this.

In 65 and later, the fill neck was low, behind the license plate. A non-vented cap was used, with the vent line on the fill neck.
64's did not have this setup, as the the fill neck was up high above the bumper, and used a vented cap

[Tom Vaught]

Originally Posted by JSchmitz

I have a '64 GTO that has always had a vented cap. Car used to run 1.6x 60' times. I've never seen any evidence of fuel coming out of the cap. Please explain your experience with this. I also rarely fill much more than a half tank.

I have seen several 64 GTOs with Vented gas caps dump gas on the track at the races.
Obviously you have never seen gas actually come out of your cap/tank when you are driving the vehicle.
And the Gas has probably vaporized by the time you finish your run, return to the pits, and get out of your car to look at the gas cap area.

Need a watcher to confirm for you that the gas does come out of the tank and how much.
AND it is a 50/50 deal on if your gas cap does leak gas.

So I would say, based on my experience from early 1965 to the 2021 that it depends.
Just like if a heater core fails due to a slug of water on a 1965+ engine that is beat on and not on
a 64 Grandma's GTO driven on Sundays.
64 GTOs can kill heater cores too, as I have posted in several threads, or leak gas.

Tom V.

[JSchmitz]

Interesting about the filler neck height differences. I can see how the later ones would be more likely. The '64 goes quit high and has a bend. I usually raced with a 1/4 tank. Hard to imagine it coming out. I forgot to mention that I installed a sump box in the middle of my tank. This would further dampen fuel slosh. Fuel leaves a pretty clear residue. I'm confident I would have seen evidence inside the fuel door area if this had happened. Good to learn more about this regardless. Maybe for future cars!

[dld]

65 used a vent tube on the neck. there was a hose that routed up to a u shape vent tube above the tag . there is a metal strap welded to rear body panel. the car used a NON vented cap

[wbnapier]

Originally Posted by dld

65 used a vent tube on the neck. there was a hose that routed up to a u shape vent tube above the tag . there is a metal strap welded to rear body panel. the car used a NON vented cap

Yes, I was curious if I could install a "T" off of this and run a return line to it?

[geeteeohguy]

Originally Posted by JSchmitz

I have a '64 GTO that has always had a vented cap. Car used to run 1.6x 60' times. I've never seen any evidence of fuel coming out of the cap. Please explain your experience with this. I also rarely fill much more than a half tank.

The OP has a '65. '65 and up GTO's have the fuel filler lower than a '64----it's behind the rear license plate, as opposed to having a dedicated gas door well above the rear bumper. '64's can run vented caps....'65-up's will leak.