Gas tank vent charcoal canister or not?
 

I have a mostly stock rebuilt 400 in my 72 Lemans. Everything on car is new or rebuilt. I am not using the intake valve assembly to control the vapors from the gas tank to the charcoal canister to carb. I was going to plumb it from canister to the port on carb. I know that would mean it is sucking vapors all the time while running. Not sure that is a good plan due to richer mixtures at times. I then thought about just leaving it open to the air from canister with a line running down below to frame. Either with the canister or eliminate the canister all together. Thoughts? Experience?

I run the charcoal canister on 2 cars here, my firebird and chevelle. They hook up slightly different but the end results is the same.

Excellent way to vent the tank without having the fuel smell. Doesn't affect the AFR at all or how the car runs in any way..

If you fail to vent the charcoal canister, it fills up with fuel, the charcoal disintegrates.

GM had a run of faulty purge valves, the carbs would fill up with charcoal.

Make sure the canister is in good condition, make sure the purge valve works. Then connect it the way it's supposed to be.

To add, you at least need to have the small 1/8" or 3/16" vacuum line hooked to ported (switched) vacuum source for the switch to activate and purge/vent the tank. If not there will be no way to vent the tank and you'll have to find other means (ie vented gas cap or baffled vent etc...) or you'll have funky run and fuel delivery issues.

I guess it doesn't really matter whether you hook the vent hose from the canister to the engine or not, you mentioned just running the line open down below the frame. That works but would be smelly. As I mentioned there is no harm in hooking it up correctly. On my chevelle it T's into the PCV and then runs to a manifold vacuum PCV connection on the carb base plate (holley). I run an AFR gauge on it and it has no affect.
On my Firebird I believe this hose goes to the vent on the upper air horn of the Q-jet.

Formulajones, Can you describe exactly how the hoses are connected on the Chevelle?

What I mean by that is: (Just an example)

1) PCV Valve is installed in Driver's side Valve Cover

2) Evap line from Canister is plumber to a tee just above the PCV Valve outlet.

3) The Gasoline Fumes from the Canister and the Crankcase Gases travel from the Tee
to the Rear Carb Nipple on the Holley carb.

Get the idea?

Question #2:

How are the lines to the gas tank hooked up. There has to be a fresh air supply to the gas tank. If so, how does the air enter the tank under acceleration?
Otherwise the fuel system/tank is trying to allow atmospheric pressure air into the tank for fuel flow to the pump and engine.

Tom V.

I assumed the purge valve worked to hold vapors back while car is started and choke is on. Car warms up and purge valve releases vent gases for burning, so too rich of mixtures don't occur. I would assume the lines not hooked to a canister, allow tank to atmospherically vent and the system performs normal.

I was going to use the port on the front bottom of the Q-Jet. Would that work?

Following are photos from my California 1970 El Camino. Vents on both upper sides of the tank tie to a single line going to the canister.

https://forums.maxperformanceinc.com...7&d=1593313383

One line from canister tied to the PCV valve to carb base,

https://forums.maxperformanceinc.com...9&d=1593313534

the other to the carb front. (White stripe). (Eldebrock carb BTW)

https://forums.maxperformanceinc.com...0&d=1593313752

There ya go, that's exactly how my chevelle is hooked up. The white stripe hose is hooked to a ported source (see how high above the throttle blades it is) that triggers the switch, which is correct.

Thanks for the great pictures.

Not sure what the canister purge flow would be when that system is activated but the way the pcv is plumbed you would not have any issues with having fewer corrosive gas vapors in the oil pan being burned properly.

Tom V.

The canister I have I bought used. It only has two fittings one from tank and one to carb. So is it safe to run a line to the carb port and let it vent there all the time.

I believe a two port canister would use a control valve like the one in the picture below with that valve controlled by the engine computer.

The vent valve lets air into the tank when cruising down the road so that you get fuel flow to the engine.
Otherwise you would be creating a vacuum on the fuel tank. NOT GOOD.

The Vent Valve would close when the engine is off, keeping the fumes trapped in the canister.
The purge valve would open at a given point in the engine cycle and the vent valve would open and remove gas vapors from the canister and excess gases from the fuel tank. then the valves would switch back into normal vent mode to the fuel tank.

Tom V.

On the '70-71 cars there is no purge solenoid or control, it just goes one port to gas tank, other port to the tank vent port on the carb.

Can you post up a schematic?

Tom V

I wouldnt use an old one without putting a small fuel filter on it. if you ever get the pellets sucked into your carb, you’ll know what I’m talking about.

That is correct, to some degree.

My 70 Formula still has 3 hoses to the cannister,
port to gas tank
vent to carb
vacuum to carb

with the difference being as you mentioned, there is no visible switch on top of the charcoal canister. That's part of why I mentioned they are slightly different between my Formula and Chevelle (70 and 72 models)

However there could be minor differences between A and F bodies anyway. Plus in 70 only California built and sold cars got this system so it was a new deal. 71 it became country wide.

Just an FYI, these canisters are reproduced and not all that expensive either. No reason to run an old one if it's integrity is in question.

Info from Tanks. I'm running vented gas caps on both vehicles for the past 7 years with no issues. The vent if used has to be located up high by the filler neck.

https://www.tanksinc.com/index.cfm?p...id=-1&mode=cat

Sorry Tom, I'm in the middle of a move and all my books are packed up. My '71-74 big cars and A-bodies have all had 2 ports only on the canister, with the '72 cars having a thermal valve on the intake to keep the fumes out of the carb before the car fully warms up. This plastic valve is usually broken after all these years... Even as late as '74 the B bodies with California emissions used the two port setup. The tank port went to a metal vent line that went back to the gas tank while the carb port went to a port on the carburetor:

http://www.slyagent.com/72gville/DSC04365.JPG

http://www.slyagent.com/74catwagon/1st-fan2.jpg

The vacuum controlled switches and solenoids didn't appear until the later '70s on Pontiac engines from the cars I've had.

You are absolutely correct, I forgot about the 2nd gen F-bodies as I've never owned one that far back.

And absolutely correct, the tanks are reproduced and there's zero reason not to replace it if you suspect yours is bad.

For those that care, if interested

I run the Tanks Inc setup in my Chevelle, and modified the tank by soldering the necessary nipples to the tank to keep my charcoal canister functional and working. Since that's how the tank vents, and it's worked perfectly for the last 35 years I didn't want to change that. I was never a huge fan of venting the tank with a nipple on top going to a baffle mounted high in the car. Always had trouble with those leaking from Tanks Inc. Just can't really find a good spot to get those vents higher than the filler neck. So a vented cap in those situations always works better if the car is pre 70 and isn't equipped with a canister.

I also recently modified a Tanks Inc setup in a 72 Formula I built in the same fashion to keep the charcoal canister working.

That's interesting Ben, I've never owned a B-body. Strange how these setups vary quite a bit between models.