Looking for an education in fuel tank venting

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71dusterdan

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Hello gents, I read somewhere that a good way to increase the size of my 71 Duster fuel tank from 16 gallons, use a 69 a body tank. They are 18 gallons. Well my tank had 4 vent tubes. The 69 I found advertised at Classic has no vent. Why is the tank vented? Why is it not vented in 69? Furthermore if it isn't vented, how did they address the issue that caused reason for a vent. Or is venting just strictly an EPA, invented issue? Yes it is a mouthful. Thank you.
 
Venting keeps the air pressures in the tank at atmosphere. I just went through this; I had a one way vented cap so it would suck air as the fuel level decreased, but it was building pressure and causing my carb to run rich by blowing fuel past the needle. I drilled a hole and welded or epoxy a piece of 3/8" fuel line a couple inches down on the top of the filler neck and ran a vent out a hole in the tail panel with a chevy PCV valve pointed up, for a roll-over valve.

It's not a big deal, if you need one, make one.
 
The tank has to be vented; one way or another. There are several reasons for this.
1) As the engine consumes fuel, the pump pulls fresh fuel from the tank. If its not vented, the tank will soon collapse, from the lack of fresh air coming in to replace the fuel going out.Its also possible for the pump to not be able to pull fuel at all, or to pull fuel that is full of bubbles.
2) On an unvented tank; on hot days the vapor pressure in there can build pretty high, sometimes high enough to greatly distort the tank, and definitely enough to be a fire hazard to the poor sob who wants to refill the tank. And in certain situations the pressure can overpower the float valve and flood the engine and oilpan, and at least 1 or 2 cylinders.This could lead to engine destruction, however improbable.

The tank can be vented to atmosphere. There are at least 2 problems with this.
1)Gasoline is made up of several constituents, of varying degrees of volatility.The first to evaporate are the ones that make your engine easy to start.
2)Gasoline costs money.

The better idea is to outfit your car with a vapor canister system, which will store the evaporating fuel for you, and reintroduce it to the engine when circumstances are right.This system will take care of the venting issue, provided that it is correctly set up.It weighs in the neighborhood of 6 to 10 pounds, Ima guessing.
 
The deal with HOW the tank is vented goes back to the early days of emissions controls.

The tanks are all vented, otherwise there would be pressure issues on hot days. The early cars vented through the fuel cap. In the late 60's the government started clamping down on fuel vapor emissions, so the automakers had to come up with a way to capture fuel vapor instead of releasing it into the atmosphere.

So, the first effort was the "4-corner" tank venting that your '71 and some '70's had. Each corner of the tank has a vent, with 4 lines leading to a vapor separator in the trunk next to the filler pipe. From there a line led forward to the engine compartment where a rubber line connected to a port on the valve cover breather. There, the excess fuel vapors were drawn into the engine crankcase and eventually burned.

That was a complicated system that probably didn't work too well, so in '72 the tank was simplified with one vent leading forward to the charcoal canister in the right front of the engine bay, which also collected vapors from the carb.

So, that's the deal with the vents. My car? I wouldn't worry about the two extra gallons.
 
71dusterdan said:
Hello gents, I read somewhere that a good way to increase the size of my 71 Duster fuel tank from 16 gallons, use a 69 a body tank. They are 18 gallons. Well my tank had 4 vent tubes. The 69 I found advertised at Classic has no vent. Why is the tank vented? Why is it not vented in 69? Furthermore if it isn't vented, how did they address the issue that caused reason for a vent. Or is venting just strictly an EPA, invented issue? Yes it is a mouthful. Thank you.
Click to expand...

My '69 Dart has no vent on the tank, nor on the gas cap. The vent is on the filler tube. there is a hose running from near the top of the filler tube down thru the trunk floor. Vents to atmosphere. I'm sure your '71 originally had the charcoal canister, etc... as mentioned above.
 
69_340_GTS said:
My '69 Dart has no vent on the tank, nor on the gas cap. The vent is on the filler tube. there is a hose running from near the top of the filler tube down thru the trunk floor. Vents to atmosphere. I'm sure your '71 originally had the charcoal canister, etc... as mentioned above.
Click to expand...


this the right answer. has a fitting on filler neck inside trunk. goes way up, makes a u, and down through trunk filler tube grommet
 
DDodger said:
this the right answer. has a fitting on filler neck inside trunk. goes way up, makes a u, and down through trunk filler tube grommet
Click to expand...
You guys are exactly right, of course. I forgot that detail completely.
 
So I have decided to use a charcoal canister set up. Now I guess I cruise the salvage yards while I look for the smallest and easiest to mount. Figure on mounting it high up on the wheel tub. Any ideas?
 
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