Venting about Venting

[jcfoote60]

OK so my Blazer tank install is almost done....I have went at this from all angles but am getting little response and seeing a few others out there with similar questions on venting these tanks. If you don't use a different fuel filler with vent tube already there and just use a bed or customer filler neck, what is being done with the large vent tube? I have just plugged the two small emission vent tubes for blazer use, but am left with the big vent tube. I had one reply that I could run it to a carbon canister....did my 72 come with one? I don't see it, just a metal line from my old saddle tank going forward to nowhere, also I got a response to just grab any canister from the bone yard...I guess I am concerned on how/where to connect the lines of a carbon canister and I plowed through 60 pages of posts and see one guy having problems with filling his tank even with the vent hose just hanging open venting to the great outdooors. The response in the thread said that until this guy moved his vent hose higher he would experience problems filling his tank.................

This is a big question mark for more than just me and although many cool conversions are going on out there with marker light fiillers, bed fillers, etc., there is litttle or no information on how and what to do with venting these gas tanks.

Trying to get good responses for all.................to benefit from.

JC

[matthufham]

well there's a vent, and an additional vent. i plugged one and the other is going to my gas filler neck which has a big hole for the incoming fuel, and a little hole for the vent. it came off a 77 chevy. up under the battery tray on my 72 i had a quadrajet charcoal canister, but i removed it since it wasn't being used and was broken.

[Mudder]

I have one vent line going to a charcoal canister that i got somewhere. Its mounted in the radiator support drivers side. The other I just ran up along side the fill tube and put one of those one way valves on it.

[pjmoreland]

If you've replaced your filler neck with one that uses a 67-70 vented gas cap, then you could just plug all of the gas tank vents and be done with it.

My 72 truck came stock with a charcoal canister mounted inside the passenger side front fender next to the battery. It has three hoses going to it. One comes from the gas tank, one from the carburetor fuel bowls and one tees into the PCV hose.

[Mudder]

A vented cap from a 67-72 is not all you need. I tried that. A Blazer tank has to be vented.

[matthufham]

Originally Posted by Mudder67

A vented cap from a 67-72 is not all you need. I tried that. A Blazer tank has to be vented.

agreed.

[71meangreenc10]

So can you get away with 1 vent line to the cannister? and running the other 2 lines on the tank together?

Smitty

[Mudder]

I think that may work. One to the canister and one that goes back up the fill tube. I think some of those vented right near the top of the fill tube.

One thing mine does that I've really been confused about is that when the tank is full it spits gas out the fill tube before it kicks the pump off.

[pjmoreland]

One of the lines that is supposed to connect to the canister comes from the PCV hose. I believe this is how the vapors that build up in the canister get removed. I'm wondering if pressure will build up inside the canister and in the gas tank if you don't connect this line.

[Johns70]

when i did my blazer tank install on my last truck i made my own filler. the large vent hose i connected to the filler a couple inches from the filler cap,pluged one small vent hoes and ran the other small vent up the side of the box to the marker light and used a rearend vent on it. this seemed to work ok. I could not fill it as fast as the pump would go but about 3/4's without pucking gas.hope this helps john

[71RestoRod]

Originally Posted by pjmoreland

One of the lines that is supposed to connect to the canister comes from the PCV hose. I believe this is how the vapors that build up in the canister get removed. I'm wondering if pressure will build up inside the canister and in the gas tank if you don't connect this line.

There is also a vacuum line that goes to a ported vacuum off the carb. This keeps fumes out of the PCV system at idle, but opens it up under throttle. I'm guessing the fumes vent out the canister filter at idle but don't know for sure.

I had wicked pressure buildup in my Blazer tank. I have to wonder if my in-tank pump for the EFI contributed to it. With a non-vented cap it about exploded when I took the cap off. With a vented cap I could hear it whistling out the filler neck. I hooked up the stock vent system, vapor tube, charcoal canister, etc... problem solved.

[71meangreenc10]

Got a pic of the carb line?

[71RestoRod]

Originally Posted by 71meangreenc10

Got a pic of the carb line?

Any oportunity for a little post whore'n.

The hose routing is nothing real exciting... It's an EFI throttle body... I teed into the one ported vacuum and into the PCV line. The third line off of the canister runs to a vapor tube in the back of the Blazer. I was actually not real excited about running more hose to the engine. It's starting to look like a late model under there.

Here's also a picture of the canister hiding under the passenger side fender... right in front of and below the sweet rad overflow container I just got from Wes. Thanks Wes!

[72402]

I put a blazer tank in a 67. It had no vapor recovery system. I used this fill tube from a 57 chevy pickup. Same size tube as the blazer tank. I plugged the small vents on the tank and ran the 1/2" vent hose to the filler. I don't know if it will work with a vented cap or not, as I converted mine to a screw-in style for the street rod gas door that I used. I'm assuming that 55-59's all have this style filler, possibly other years too.

[matthufham]

mine is exactly like that from a 77 chevy. ran the vent to the filler neck.