Charcoal / vapor / evap canister, how it should be set up and other information

vphouger said:

Which shop do you manage? I might have to drop in sometime. I did order the canister off ebay. It should be here early next week. Given I've only owned the truck for about 2 and a half years, I discovering more "bubba-isms." All of the emissions are deleted.

Good to know you aren't that far away.

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I manage Luke's Redmond Automotive. You are absolutely welcome to stop by sometime, but if we happen to be busy when you drop by we might not get much talk time, but I'll turn you loose to look around the shop especially if we have any interesting classic projects going on. Just tell whoever is at the front desk that your a friend of Rob's. Bonus points if you drive your truck and I get to look at it!

Really great, I don't understand how some folks are ok with not evap system, no air pump and no cat. I tell you I'm

SirRobyn0 said:

I manage Luke's Redmond Automotive. You are absolutely welcome to stop by sometime, but if we happen to be busy when you drop by we might not get much talk time, but I'll turn you loose to look around the shop especially if we have any interesting classic projects going on. Just tell whoever is at the front desk that your a friend of Rob's. Bonus points if you drive your truck and I get to look at it!

Really great, I don't understand how some folks are ok with not evap system, no air pump and no cat. I tell you I'm

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I did run the truck with the gas cap off - ran terrible. I received the new canister and am hooking it up. Now where are those instructions you provided?

vphouger said:

I did run the truck with the gas cap off - ran terrible. I received the new canister and am hooking it up. Now where are those instructions you provided?

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Page 1, let me know if you have questions... I hope that's your problem. But if it ran bad with the caps off then I doubt the canister will fix it, but at 40 years old I'm sure your old canister is not in the best of shape anyhow.

For Sir RobynO - my ‘75 K25 still has the original two port canister, and with the original Quadrajet there was a dedicated port for the canister vent line.

When I switched to a newer carburetor I first just put a T in the PCV line to the carburetor and connected the canister vent there, but it seemed to run quite lean at cruise (I have a wideband AFR gauge). I now have the vent line disconnected and left it open in the engine compartment. Truck seems to run fine, but I know the vent should connected somewhere that pulls the vapor.

Where should I connect the vent line for a two port canister? Do I need an external purge valve to control it?

Thanks for any input.

SirRobyn0 said:

Page 1, let me know if you have questions... I hope that's your problem. But if it ran bad with the caps off then I doubt the canister will fix it, but at 40 years old I'm sure your old canister is not in the best of shape anyhow.

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I'm thing the same thing. Thanks.

75gmck25 said:

For Sir RobynO - my ‘75 K25 still has the original two port canister, and with the original Quadrajet there was a dedicated port for the canister vent line.

When I switched to a newer carburetor I first just put a T in the PCV line to the carburetor and connected the canister vent there, but it seemed to run quite lean at cruise (I have a wideband AFR gauge). I now have the vent line disconnected and left it open in the engine compartment. Truck seems to run fine, but I know the vent should connected somewhere that pulls the vapor.

Where should I connect the vent line for a two port canister? Do I need an external purge valve to control it?

Thanks for any input.

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Do you need an external control valve. It's not absolute requirement, but it really should have one and might solve your problem.

On a 5 or 6 port canister the control valve part is built in. If you run a 2 port without a control valve it will purge anytime the engine is running. An external control valve is going to have one line going to ported vacuum, which will prevent purge at idle and just off idle, until there is good vacuum at the ported port. It'll also cut the purge as you step on the gas sooner than it would on it's own. Most external control valves need to see around 10 inches of vacuum on the ported line before it'll allow purge. So if you are seeing that lean AFR at very light throttle, before ported would be fully open then adding an external control valve would do the trick.

Connecting the purge line to the PCV line is the correct way to do it.

What you really want is this >>>> https://a.co/d/4wtwQQG <<<< connect it into the 3/8" purge line and connect the smaller "control" line to any ported source. Since fuel vapor does not travel though the control line it can be tee'd into any ported source and it does not matter if it shares that ported source with other devices. This is really the correct way to set it up.

Oh, also running around with the purge line disconnected as you currently are is fine for a while, but eventually the canister will load up with fuel vapors, it'll kill the charcoal and get smelly - timeline on that is hard to say.

Hopefully that helps, let me know if you have questions.

Sounds like a good alternative to try. My engine runs about 19-20" of vacuum at idle (fairly mild cam), so it will definitely have enough vacuum to signal and close the purge line at idle. I don't have a vacuum gauge inside the truck, so I'm not sure what the vacuum is at cruise.

I run my distributor advance off the full vacuum port, so the ported vacuum is capped on the carburetor. It's an easy fix to add the purge valve control, and maybe it will help with the lean cruise AFR.

Thanks again.

We build trucks and vehicles here in Arizona. We use a Vapor Trapper but it goes right to atmosphere. With all the EFI and LS swaps going on, a lot of these trucks have had their canister discarded. But lets be honest, the charcoal would no longer be good in them anymore. The only thing we would like to add is the ability to add the purge valve. Currently we just vent them to atmosphere and this works very good to eliminate the smell and address the moisture issue like another poster had mentioned. We like the Vapor Trapper because it is a nice part and it is serviceable so you can change the charcoal once it gets saturated.