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#1
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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?
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#2
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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.. |
#3
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Thanks for all the info and insight. PontiacJim, thanks for the diagram. I looked in my manual and have that. So, the A body stand pipe allows for the tank to vent. My issue is, I did not have and was not planning on using the purge valve that would've been in my intake. I installed an after market thermostat for a gauge there. If I were to run the canister to the carb port, it would be sucking vapors at all times and possibly messing with my idle. If this line to carb was left open to the atmosphere and hanging down by frame, it could still vent fumes I assume, but maybe smell a little at times when parked. Am I wrong or is there a better way around this without the purge valve in intake?
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#4
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Quote:
Only hoses to the engine........I simply run the 3/8 hose through the PCV as pictured which goes over to the base of the carb. Then the little vacuum line runs along with it, also to the carb, and is ported vacuum at the metering block (holley carb) to trigger the vacuum switch on top of the charcoal canister. That's it. It absolutely has no affect what so ever on idle or part throttle cruise. My air fuel ratios are very steady and spot on. |
#5
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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. |
#6
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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. |
#7
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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.
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"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. |
#8
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I don't think my canister has a switch in it. It is flat on top and if I blow into it, it takes air and not plugged. One person mentioned if running all the time, it would create too much vacuum for the tank and cause a problem. Not sure if that can happen with the stand pipes. Maybe they allow enough source for excess vacuum so as not to suck the tank in. I thought the stand pipe would allow atmospheric air pressures to fill the tank as needed.?
400 Lemans, thank you for that insight. |
#9
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My Formula doesn't have a visible switch on top of the canister either. It's hooked up in a similar fashion, a vacuum line and 3/8 hose to the carb. There is no purge valve on the intake of that one either.
As far as I know, both systems on my Chevelle and Firebird run all the time, they've never created too much vacuum at the tank or created any kind of issue. In fact without the tank venting the opposite problem occurs. Remember the vacuum connection is switched (ported) at the carb so it's not active at idle anyway. Matter of fact, I would think you'd want it functioning most all the time anyway, because a tank that isn't vented at all causes way worse issues than a charcoal canister ever will. I've had that issue before and fuel delivery becomes a nightmare. |
#10
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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? |
#11
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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.
One line from canister tied to the PCV valve to carb base, the other to the carb front. (White stripe). (Eldebrock carb BTW)
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Ed 1968 GTO (Thanks Mom) 2006 Silverado 2007 Cadillac SRX 2015 Chevy Express 2024 Cadillac LYRIQ |
#12
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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.
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#13
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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.
__________________
"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. |
#14
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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.
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The Following User Says Thank You to tjs72lemans For This Useful Post: | ||
#15
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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.
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"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. Last edited by Tom Vaught; 06-28-2020 at 07:05 PM. |
#16
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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.
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#17
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Quote:
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. |
#18
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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.
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#19
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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:
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. |
#20
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