Carbon Canister Venting Question

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Feb 04, 2012 06:19:14
MGB7509
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Hey guys!

I looked and looked for an answer on here but couldn't find the exact question.

My 1975 B was stripped of all the emissions except the carbon canister. Now, my question is, if you have the vent tube going to the valve cover from the Carbon Canister do you need to have a vented filler cap? I just realized that I have both a vented cap and the vent line running to the Carbon Canister?

Any help would be awesome!

Zach S.

Feb 04, 2012 06:43:19
Steve Lyle
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No, you don't. Your gas tank is supposed to be vented to the canister, and have an unvented filler cap.





Feb 04, 2012 06:45:23
MGB7509
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By filler cap I meant the oil filler cap on top of the valve cover.

Sorry!

Feb 04, 2012 08:27:26
davester
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The answer is still no. The charcoal canister vents the valve cover. However, it is also important to have the side cover vented to the intake side so that a slight vacuum is maintained on the crankcase, thereby extracting corrosive gases and replacing them with makeup air pulled through the canister. If your DPO has monkeyed with the emissions plumbing then the crankcase vent may be messed up too.

Feb 04, 2012 09:05:51
MGB7509
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Thanks Dave! I'll have to check out the side cover vent.

Feb 04, 2012 10:09:10
forestghost07
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Hi Zach, if you wanna use a nice vented chrome oil cap instead of the ugly OEM one just plug the cap's vent hole ... I did a little tapping screw underneath and flowable RTV silicone windshield sealer on top; can't see it and it worked :)

Feb 04, 2012 10:14:18
MGB7509
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Quote: "
The answer is still no. The charcoal canister vents the valve cover. However, it is also important to have the side cover vented to the intake side so that a slight vacuum is maintained on the crankcase, thereby extracting corrosive gases and replacing them with makeup air pulled through the canister. If your DPO has monkeyed with the emissions plumbing then the crankcase vent may be messed up too.
"


Dave,

I have a Weber 32/36 DGV setup and during the install the side cover hose was not needed. I think I ended up just routing the old hose down to the bottom of the car. Anyone with a Weber 32/36 setup tell me where you vented the side cover to?

Feb 04, 2012 11:52:08
Steve Lyle
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Quote: "
The charcoal canister vents the valve cover."



No, it doesn't. It works the other way around. The vacuum from the intake, through the side cover, pulls air from the canister, containing whatever hydrocarbons are trapped there, into the valve cover, through the pushrod holes and through the side cover vent.

John Twist has a video on this topic.

Feb 04, 2012 19:57:14
davester
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Quote: "
[quote=davester,1975625,1975725]
The answer is still no. The charcoal canister vents the valve cover. However, it is also important to have the side cover vented to the intake side so that a slight vacuum is maintained on the crankcase, thereby extracting corrosive gases and replacing them with makeup air pulled through the canister. If your DPO has monkeyed with the emissions plumbing then the crankcase vent may be messed up too.
"


Dave,

I have a Weber 32/36 DGV setup and during the install the side cover hose was not needed. I think I ended up just routing the old hose down to the bottom of the car. Anyone with a Weber 32/36 setup tell me where you vented the side cover to?
[/quote]

I'm pretty sure the weber setup instructions indicate that you should connect the side cover hose to a fitting on the air cleaner. Venting it to the bottom of the car is not the best idea. In the olden days (pre-1950s) they used to do this (it was called a road draft tube) but had the tube positioned and cut so as to pull a slight vacuum on the crankcase. This method was abandoned in favor of positive crankcase ventilation both because it helped scour contaminants out of the crankcase and because it cut down significantly on smog emissions. It's important to have a vacuum in the crankcase because otherwise corrosive combustion gases build up in the oil and cause damage to your bearings, etc.

Feb 04, 2012 21:08:55
cptmoney
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What's the best move for cars with the unported SU HS4 carbs? How does one go about recreating that crankcase vacuum? I think we'll just put a breather on our front tapped cover vent tube - but that's still not creating the vacuum, just letting the crankcase gases vent out. What should we do?

Feb 05, 2012 09:17:13
davester
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Quote: "
What's the best move for cars with the unported SU HS4 carbs? How does one go about recreating that crankcase vacuum? I think we'll just put a breather on our front tapped cover vent tube - but that's still not creating the vacuum, just letting the crankcase gases vent out. What should we do?
"


Prior to ported SU HS4s, MGBs used a PCV valve attached to a port on top of the intake manifold. You could buy a period PCV valve or just buy a generic one.

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