Whats inside the gas tank? At evap barb.

[Ben A...]

Hey folks,
Need a bit of help understanding the gas tank/evap system on my 1973 Duster.

The carbon can is gone, and I want to repurpose the line running back to the tank as a fuel return line. No fear, I have already modified the filler neck to vent like the earlier years.

I plumbed up the bypass system and when I started the car fuel started dumping out of the "return line" at the tank.

So i'm wondering... The barb fitting (in photo) at the top edge of the tank. What is behind that, inside the tank? I ran a wire in there to check for blockage and its quite tight, but the wire does go in-- though it snags on things.

Can I use that barb to return fuel to the tank? It seems as though there is an orifice in that nipple, and then something behind that? Am I wrong?

Im really hoping not to drop the tank and install my own input. Thank you for any help.

Photo shows the barb (circled) and the end of the evap line-- I removed the tubing to barb for photo.

Ben

 

[Murray]

Am not sure of the answer but do know that originally that line returned vapor not fuel.

 

[Ben A...]

Am not sure of the answer but do know that originally that line returned vapor not fuel.

Thanks Murray-- Yeah thats why I think theres something behind it and perhaps not a direct inlet that I could use for a fuel return..

 

[67Dart273]

You can NOT use that line for fuel return (or shouldn't) for a couple of reasons

First, originally, that line IS the tank vent. It went up front as you know, tied in with the evap/ carbon can system, and vented the tank. THERE IS NO OTHER vent

Also, did your car originally have the "big can" (vapor separator) in the trunk? It should not have had, unless it was an earlier model. Earlier evap systems had that, with 3? or 4? vent lines from the tank, later ones had only one 1/4" fitting at the tank

Look up a 69 service manual (free) from MyMopar and you will quickly see how the "un" evap tanks vented----using a simple tube welded into the upper filler neck

(While you are at it get a 73 manual from there too. Read the fuel and emissions sections to find out how this all worked)

Second, that line is likely WAY too small if you are talking about a return regulator/ EFI/ etc. If you are simply talking about a fuel/ vapor return from something like a 3rd port fuel filter with the anti-vapor lock orifice, then it will be ok, BUT YOU STILL would need to provide a vent

The fuel caps in none of these cars are vented--and that fact helps keep fuel off the fenders

If you don't provide another vent, that fitting needs to remain "up front" and "up high" to prevent siphon.

This is 70 / without evap and 69/earlier tank vent

 

[Ben A...]

You can NOT use that line for fuel return (or shouldn't) for a couple of reasons

First, originally, that line IS the tank vent. It went up front as you know, tied in with the evap/ carbon can system, and vented the tank. THERE IS NO OTHER vent

Look up a 69 service manual (free) from MyMopar and you will quickly see how the "un" evap tanks vented----using a simple tube welded into the upper filler neck

(While you are at it get a 73 manual from there too. Read the fuel and emissions sections to find out how this all worked)

Second, that line is likely WAY too small if you are talking about a return regulator/ EFI/ etc. If you are simply talking about a fuel/ vapor return from something like a 3rd port fuel filter with the anti-vapor lock orifice, then it will be ok, BUT YOU STILL would need to provide a vent

The fuel caps in none of these cars are vented--and that fact helps keep fuel off the fenders

If you don't provide another vent, that fitting needs to remain "up front" and "up high" to prevent siphon.

Hey 67Dart273,
I was hoping you would see this as I've read many of your posts regarding this! Thank you.

I did modify my filler tube with a vent line.

I had read people were using the vent barb at the tank as a return from Wix type orifice/vapor filters. But I im running a return from a Quickfuel bypass regulator and I guess its pushing way more fuel than the barb input at the tank can handle. Dang.

(Just running a mechanical pump)

Dang
So I need to either make my own return input or enlarge the current barb at the tank...?

Thank you for the help!

 

[67Dart273]

I would. There are some sender units made (aftermarket) with a second fitting. You can measure the backpressure if you can rig a tee fitting and gauge You (I'm sure) know how I'm "redneck" On my 67 I took an awl and punched (not drilled) a hole into the drained tank, then filled the tank with CO2 from my welder bottle. Then just silver brazed a scrap of copper in there. Use your own judgement.

 

[Ben A...]

I would. There are some sender units made (aftermarket) with a second fitting. You can measure the backpressure if you can rig a tee fitting and gauge You (I'm sure) know how I'm "redneck" On my 67 I took an awl and punched (not drilled) a hole into the drained tank, then filled the tank with CO2 from my welder bottle. Then just silver brazed a scrap of copper in there. Use your own judgement.

I look forward to anytime I can use an awl on my car.

Thank you again.
Guess ill just lay under there and do
some thinking.

Regarding sending units... something like this? Is the second nipple (1/4) high enough for a return line? Seems low... But perhaps that doesnt matter.
Right Stuff Detailing YSU6901 Right Stuff Detailing Fuel Tank Sending Units | Summit Racing

 

[67Dart273]

^Yeh. Notice the return is also 1/4 but it is short. You could use larger tube to the tank, and step it down right before. This will reduce backpressure a bunch.