Fuel tank vent / breather questions.

[demon322]

Hello all. My 71 Demon is nearing completion at the paint shop, so it’s time for me to come up with a plan for the fuel system.

The car was originally a slant 6, and it must have had the large vent tube in the trunk originally (that I think went with a tank with the 4 vents.) All that was gone when I got the car.

I now run a 340 and installed an aftermarket tank. I possibly have the wrong (aftermarket) sending unit also as mine has a port that is just plugged off (could this have been a return line in some applications?)

My question is, can I run this tank and hook up the vent line circled in yellow here to a breather on the valve cover? This second line here on the fender has not been used since I’ve had the car but I think it goes back to the vent on the tank.

Orange car is not mine and probably a pic I found on here, just showing the breather setup I am asking about. Thank you.

 

[CRUZE 418]

I am not sure how a '71 was vented, but I certainly wouldn't vent it through the engine like the orange picture.

 

[jbc426]

You can run a vented/anti-surge cap plus a feed and same sized return to the factory sending unit. I even added an electrical bulkhead fitting to install an intank pump. Then run a return fuel pressure regulator soft mounted in front of the engine.

 

[Yote]

I could be mistaken but I believe that line runs to
The charcoal canister.
Yote

 

[CRUZE 418]

It does, on my '72, but he's got a '71. Not sure what a '71 had.

 

[demon322]

I could be mistaken but I believe that line runs to
The charcoal canister.
Yote

Not saying you’re wrong but I can’t find any info on that (charcoal canister) for a 71.

 

[67Dart273]

Look running a vented cap is stupid. You are risking gas over the paint of the quarter. The object in the trunk on the early tanks was a liquid / vapor separator. Probably the easiest way out of your problem is to modify your filler tube to "be like" a 69/ earlier which had not evap controls. You do NOT want to use the port in the sender because it is below the liquid level. That port was NEVER on any OEM sender. It's purpose is for fuel return AKA EFI systems, etc.

Read this, or read your shop manual. These systems AND the filler necks and caps changed almost every year from 70 on

Fuel Tank Venting Question

The 69/ earlier vent. What is being depicted is, the tube goes up high against the top of the quarter fender and then back down along the filler tube, through the gasket in the trunk floor, and ends, open ended, inside the frame rail. This causes a siphon break, and will only drain fuel if heat expansion drives fuel clear up into the neck.

The thing you will have to be careful of, using your late model neck, is that because if the protruding vent nipple, the 69/ earlier tube installs from INSIDE the trunk, with the tank going in last

The later ones with evap and with no vent tube, install from the OUTSIDE, so a vent fitting will hit the sides of the hole. You can get buy if you flush braze/ weld a bushing into the tube wall so it will clear, going into the hole, then install a hose/ tube fitting after

 

[demon322]

It does, on my '72, but he's got a '71. Not sure what a '71 had.

I wonder if the charcoal canister started in 72. Mine just had the canister in the trunk from the 4 line system which was long gone when I got the car. But the hole in the trunk is still there. From the 71 FSM it references a vent line to the breather in a couple of places (as well as a 3rd line from the breather cap to the carb which my aftermarket carb won’t have I assume) but does explain why some 71 breather caps I see have 3 ports possibly.

 

[demon322]

Look running a vented cap is stupid. You are risking gas over the paint of the quarter. The object in the trunk on the early tanks was a liquid / vapor separator. Probably the easiest way out of your problem is to modify your filler tube to "be like" a 69/ earlier which had not evap controls. You do NOT want to use the port in the sender because it is below the liquid level. That port was NEVER on any OEM sender. It's purpose is for fuel return AKA EFI systems, etc.

Read this, or read your shop manual. These systems AND the filler necks and caps changed almost every year from 70 on

Fuel Tank Venting Question

The 69/ earlier vent. What is being depicted is, the tube goes up high against the top of the quarter fender and then back down along the filler tube, through the gasket in the trunk floor, and ends, open ended, inside the frame rail. This causes a siphon break, and will only drain fuel if heat expansion drives fuel clear up into the neck.

The thing you will have to be careful of, using your late model neck, is that because if the protruding vent nipple, the 69/ earlier tube installs from INSIDE the trunk, with the tank going in last

The later ones with evap and with no vent tube, install from the OUTSIDE, so a vent fitting will hit the sides of the hole. You can get buy if you flush braze/ weld a bushing into the tube wall so it will clear, going into the hole, then install a hose/ tube fitting after

I don’t think my cap is vented, this is what it looks like. And I don’t think my filler neck has a vent like the one you describe. I will be with the car tomorrow and will check to make sure.

You confirmed what I was thinking about the sender, that the extra port is for a fuel return which I don’t think I need. So can I just leave it plugged as shown in the picture or would you replace it with a different sender?

So the replacement tank I have has a (I assume) vent connection on the top center. That is the line which I think runs to the front fender and I was thinking about connecting to a two port breather cap.

 

[CRUZE 418]

I bought a new '71 340 Demon back then. Do not remember any of this!