Fuel tank vent

[ERIK D]

Need a little help with the fuel system on the bullet.as u guys know the bullet is 99% done, and when i am not driving it ,it sleeps in the garage,but every time i need to get to the garage,there are alot of gas fumes in there.i have already checked all the fuel lines and there is not one drop comming out,also the carb is dry on the outside,when its running it idles just fine and the throttle response is great,so i dont think is the carb.maybe someone can second me on this,i've been thinking that it could be the fuel tank vent,i did put one in.it is a steel braided line with a filter on the end.maybe there's another type i should use.
You all's help w/ this problem will be greately appreciated.

[texanidiot25]

Charcoal filters get rid of the fumes much better, and basically that's what modern cars use. I think the 71 and 72 used them as well.

[68caliRat]

The vent's purpose is not to allow the free exchange of air from the tank, but rather to allow air to escape as you fill your tank, or enter as the fuel is used. Modern vehicles have a system that traps the fuel vapor and returns it to the tank while still allowing air to enter or leave the system, while fueling. That vent is supposed to be hooked up the tank filler to allow the gas station's pump to trap the vapor as you fill up. Otherwise air gets into the tank through the gas cap. As fuel is used the negative pressure opens the valve in the gas cap to allow fresh air to enter the tank keeping the inside and outside pressures equal. I'd just cap it, or hook it up as designed right to the filler next to the cap.

[ERIK D]

Originally Posted by 68caliRat

The vent's purpose is not to allow the free exchange of air from the tank, but rather to allow air to escape as you fill your tank, or enter as the fuel is used. Modern vehicles have a system that traps the fuel vapor and returns it to the tank while still allowing air to enter or leave the system, while fueling. That vent is supposed to be hooked up the tank filler to allow the gas station's pump to trap the vapor as you fill up. Otherwise air gets into the tank through the gas cap. As fuel is used the negative pressure opens the valve in the gas cap to allow fresh air to enter the tank keeping the inside and outside pressures equal. I'd just cap it, or hook it up as designed right to the filler next to the cap.

the problem is that i can't hook it up to the filler neck cause the tank has a very short one.

[chevy_mike]

Originally Posted by 68caliRat

The vent's purpose is not to allow the free exchange of air from the tank, but rather to allow air to escape as you fill your tank, or enter as the fuel is used. Modern vehicles have a system that traps the fuel vapor and returns it to the tank while still allowing air to enter or leave the system, while fueling. That vent is supposed to be hooked up the tank filler to allow the gas station's pump to trap the vapor as you fill up. Otherwise air gets into the tank through the gas cap. As fuel is used the negative pressure opens the valve in the gas cap to allow fresh air to enter the tank keeping the inside and outside pressures equal. I'd just cap it, or hook it up as designed right to the filler next to the cap.

Ummm.... not true. The vent's sole purpose is to ALLOW air to enter to enter when there is a vacuum created in the tank by the fuel pump draining it AND to allow air to be displaced when you are filling the tank and the fuel is pushing the air to go somewhere.

Old car systems just vented the tank to the open air, which gives you the wonderful smell of gas if kept in a closed garage. Back in the 60's and earlier, a lot of homes did not have closed garages so it wasn't a concern.

Getting into a more modern system of the 70's, they started using charcoal canisters and vacuum tubing up to the carbs, to help draw the fumes into the engine and not vent into the air. It worked okay but still on hot days, the fuel in the carb would actually off gas and you would get some smell, not as bad as an open system though.

Now modern cars started using vacuum systems that would purge the systems into a closed loop setup, which never actually allows the fumes to vent into the air. This added a lot of complexity to the systems but they worked great. No more fumes.

In CA, they started testing these newer systems and you will fail a smog check if it doesn't pass the test of the evap system. All it takes is one vacuum leak and the system doesn't work.

In your case, you are going to need to vent the tank into a charcoal canister to really have any chance of gettting rid of this problem. Other methods, like longer vent lines, check valves and such will help some but not 100%.

My .02 worth.

[ERIK D]

Originally Posted by chevy_mike

ummm.... Not true. The vent's sole purpose is to allow air to enter to enter when there is a vacuum created in the tank by the fuel pump draining it and to allow air to be displaced when you are filling the tank and the fuel is pushing the air to go somewhere.

Old car systems just vented the tank to the open air, which gives you the wonderful smell of gas if kept in a closed garage. Back in the 60's and earlier, a lot of homes did not have closed garages so it wasn't a concern.

Getting into a more modern system of the 70's, they started using charcoal canisters and vacuum tubing up to the carbs, to help draw the fumes into the engine and not vent into the air. It worked okay but still on hot days, the fuel in the carb would actually off gas and you would get some smell, not as bad as an open system though.

Now modern cars started using vacuum systems that would purge the systems into a closed loop setup, which never actually allows the fumes to vent into the air. This added a lot of complexity to the systems but they worked great. No more fumes.

In ca, they started testing these newer systems and you will fail a smog check if it doesn't pass the test of the evap system. All it takes is one vacuum leak and the system doesn't work.

In your case, you are going to need to vent the tank into a charcoal canister to really have any chance of gettting rid of this problem. Other methods, like longer vent lines, check valves and such will help some but not 100%.

My .02 worth.

great response.can u elaborate a little more on the last paragraph?
You might have the solution to my problem.
Thx

[chevy_mike]

Originally Posted by ERIK D

great response.can u elaborate a little more on the last paragraph?
You might have the solution to my problem.
Thx

You'll need to find a charcoal canister out of some vehicle at the junk yard. Find one that is not full of fuel (sloshing sound). Ideally something from the 70's as most of them have only an input and output connections, which is all you need. You will want to mount it some place so that the connections are higher then the top of your fuel tank. Then plum your vent line from the tank to the input connection on the canister. Then either leave the output connection open or run a line up higher. The charcoal in the canister will help filter the fumes. You can also run the output up to the air cleaner (to an area inside of the air filter) which will "pull" the fumes out of the tank but without enough vacuum that it causes the tank not to vent. This would be a semi sealed system and have your best chances of not having the fuel smell.

BTW, with this setup, you want to run a non vented gas cap.

Hope that helps.

[68caliRat]

Then venting won't be a problem as air will transfer to the filler without it. I'd say cap it, unless somebody else has a good reason not to. I have to ass-u-me that this has been handled before on many occasions. But I have no practical knowledge as mine is still behind the seat.

[ERIK D]

if i cap it,wouldnt the pump burn since by doing so,it will create a vacum in the tank,thus the flow of gas be restricted?

[67ChevyRedneck]

The problem I see with your set up is that you have an open air filter. That's where your smell is coming from. You're basically venting your fuel tank into the air, hence the smell.

What I did was use a motorcycle style vent that has a check valve in it. It will allow air to enter the tank when fuel is being sucked out (this will allow air to displace the fuel leaving the tank and not create a vacuum). I installed mine in my bedside so it was plenty high on the truck.

You also need a vent to let air out of the tank when it is filled, this is normally accomplished with a vent line on the filler neck. I used the door/filler neck off of a 88-98 silverado so that problem was solved for me, I can't see in the pic on how you plan to vent your filler neck, or where it will be located.

What I used:







You can't really see it, but it IS in there.

[ERIK D]

looks like this is gonna be very simple solution to my smelly problem.
thank you all very much for the responses.
i will first do the check vave mod,and hopefully i wont hear a thing from the wife anymore.
if that doesnt do it then i will do the cannister thing,and if that doesnt do it,i will be forced to build aseparate garage for just the bullet.(which i will end up doing anyway).
thanx

[67ChevyRedneck]

This is where I got mine:

http://cgi.ebay.com/ebaymotors/Parts...3A1|240%3A1318

It's $9.95, free shipping, and seller has 100% feedback on ~2,300 sales

Description:

Replaces awkward fuel tank breather hoses. One way vent allows the tank to breath while preventing fuel from splashing out and making a mess.

This has been on my truck for almost 3 years and I've never had any issues with fuel.