My duster originally came with a gas tank that has 4 vents, with hoses that run to a canister (fuel vapor separator is its official name I think) in the trunk, and the canister output to the breather on the valve cover.

When installing the efi setup, I capped that line off at the canister and used part of it as a return for the pressure regulator. I figured now its time I do something and get a proper vent system in order, especially seeing as I use a locking gas cap (all locking ones are non-vented, right? pretty sure my original vented one was stolen). That, and I'm having some fuel delivery issues I'm not able to easily replicate and I have to start somewhere....

Whats the best way to go about it? Splice all the vents together and use a charcoal canister, or cap off 3 of the vents and just use one... or maybe replace the tank all together with a later model one that just has one vent?

Speaking of charcoal canisters, are we limited to junkyards for them? None of the parts stores online seem to list them any more. Or maybe one off a completely different vehicle? Read a previous post on here about the filter being difficult to replace on the original mopars.

How do those things work anyway, I couldn't find a clear explanation online. No clue on how to hook one of those things up.

Or am I completely off base and should go another route to vent the tank?

OK, if we're talking about a 4-vent tank and a fuel vent fitting on the crankcase breather, it's probably a '71 (Federal/Canada) or '70-'71 (California) car. These cars originally did not have a charcoal canister; those didn't start until 1972. See my old post here for more detail on the '70-'71 system compared to the '72-up system. It's helpful to understand how the system was originally configured before you start modifying it.
No. Vented caps aren't used on Mopars of any year. The correct cap for your application, whether it be a locking or non-locking cap, is a "pressure-vacuum" cap. That's a special kind of vented cap, but we don't call them "vented" because a "vented" cap specifically means one that has a plain old hole in it, without valves or any other kind of regulation to the venting. A pressure-vacuum cap has a double-ended spring-loaded valve in the centre. It allows air into the tank when dropping fuel level lowers the air pressure above the liquid fuel, but does not allow the free escape of fuel vapours. If vapour pressure above the liquid fuel goes over a certain value, the valve does open and allow the vapours to escape. This value is set high, since the evaporative emission control system (ECS) is supposed to handle these vapours. It is only in unusual circumstances that the overpressure relief valve in the cap opens and allows vapours to escape. These conditions include damage or misconfiguration of the ECS, or a sudden reduction in atmospheric pressure (as when rapidly climbing a mountain) causing such a large volume of vapour that the ECS cannot handle it. So, check to make sure you have a working pressure-vacuum cap on your car.

You will not need to worry about carburetor bowl venting, since you no longer have a carburetor. That makes life easier. All you have to do is vent the tank properly and safely. Here's my prescription:

Retain the factory "4 into 1" fuel vapour separator in the trunk. It may be necessary for you to remove the separator and blow it out until there is no more liquid fuel in it, since it sounds as if you've been running your liquid fuel return to it for awhile. Leave all four corner hoses connected to this device. The single hose that runs from this device to the engine bay should be connected to the large-diameter "TANK" port of a new or good used charcoal canister. The large-diameter "PURGE" port of the canister should be teed into the PCV valve hose. The small diameter purge valve control port of the canister should be teed into the distributor vacuum advance hose. This way, the system will work as designed, admitting fuel vapour to the engine for burning only above idle.

New charcoal canisters are available. It's just that they've been N/A in the aftermarket for Mopars for quite some time. GM and Ford canisters are still readily available (and if I'm not mistaken, I have a line on a bunch of brand new ones at very good prices — checking into it!). All canisters do the same thing, the only differences are the number of ports, the presence/absence of a purge control valve, and the size/shape of the canister.

Whatever canister you wind up using, there needs to be a canister purge valve that only opens when the engine is above idle. Some canisters have these built in (looks like a round "flying saucer" atop the canister itself), but many do not, including Mopars after '72 and most late-model ones in which purging is controlled by the computer via a solenoid. Fortunately, standalone purge valves are readily available. NAPA Echlin # 2-28011:



Small fittings gets teed to the vacuum advance hose (which should have no vacuum at idle, full vacuum above idle)

Big fittings go inline with "PURGE" hose from canister to PCV hose.

The reason there's no purge valve on most of the Mopar canisters is that the "purge" port on the original-equipment carburetors were set up so as not to have vacuum at engine idle.

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Too many people who were born on third base actually believe they've hit a triple.

Yep, its a 71 california car.

I'm not running fuel through the seperator now - I capped the hose going from the seperator to the crankcase, at the seperator, and used that hardline to tee back into pump input for the return. I may later redo this and get the sender with two ports on it. But thats a seperate issue, I'll take out the seperator and clean it.

The cap I have now is a stant locking unit that appears to have some sort of valve in it. I have no idea if its the right one for the car or not. Has no part # on it. Any idea which cap is the right one? Kragen.com lists several stant #'s - 11572 and 17572 descriptions are "to late 71" and 10572 described as "late". Are the valves in the cap set differently amongst the 70-71 cali cars and the later 70's cars? If you have any part #s, stant, fram, gates etc of whats the right locking cap that would be cool.

Is there any solution that does not require ported vaccum? The throttle body I have now does have ported vaccum ports, but I may get rid of it soon enough if I ever get the EDIS system going.

Figured.
Oh, OK, gotchya. No need to worry about the separator much, then, though cleaning it would not be a bad idea.
Or just drill and tap in a return fitting onto your existing sender's flange plate...

Then it is probably a pressure-vacuum unit.
There'd be a definite difference between '70 Fed/Can and '70 California cars ('70 Fed/Can would use the '69-type cap, '70 California would use the '71-type cap). I'm not too sure about the differences between early and late '71. 10572 is also called for on '72-'76 cars, so I can certainly buy their being used in at least some of '71 production. Lacking specific data, I'd guess(!) that '70 California plus '71-'76 Fed/Can/Cal caps are all interchangeable.
Yes, do you have any kind of a throttle position sensor or idle switch in your setup?

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Too many people who were born on third base actually believe they've hit a triple.

Talking about charcoal canisters, how much difficult is to install one in a car that never had one?, I was thinking about installing one in my 71 Duster, but don´t know what I need to install it, you know, my original gas tank works or I need one?, and how they are connected.

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Mopar Muscle is well defend by Slant Six!

Valiant Duster 1971

Avenger - the details in Dan's second post would apply to your car as much as mine interms of hookup detail - my car never had a charcoal canister either. As long as your tank has a vent tube.....

Dan - yea the megasquirt has a TPS sensor, but I'm not sure if there is a way to fire off an output relative to TPS. No idle switch or solenoid. I haven't seen every car in the world but all modern cars I've seen don't have any ported vaccum ports. I wonder how they do it?

Is there brand preference as to gas cap? The picture of the locking fram one on kragens website looks like it may be easier to get on and off because of the "handle" on it, but I'm not sure if their gas cap construction is as bad as their oil filters.

Keep me in the loop about those charcoal canisters.

I've read some references to charcoal canisters w/o purge valves. Whats the deal with these - is it like ones with a purge valve thats stuck open, and you have to calibrate engine at idle to compensate for the vaccum leak?

I'm trying to avoid having to run another line to the engine compartment. For short term fix I'm just looking to vent the tank.... maybe some sort of charcoal filter I can mount near the tank and have its "output" left open?

Fellow efi users - what does your tank vent setup look like?