FUEL TANK CAPS:

[Stewart G Griffin]

Let's not be concerned about why i need a new cap, let's just be concerned about the following questions, ok?

1)Are the caps on 73-86 carbureted trucks vented?

a) If so, this basically means i can interchange it with virtually any other GM car or go to the "HELP" section of the auto parts store and get a generic replacement?

2) Explain, in detail, how and the purpose for the venting system on fuel tanks, either by the cap or other means.  Thank you.

[VileZambonie]

No they are not vented to the atmosphere. Fuel tank vapors are directed into the charcoal canister and back into the engine and burned in combustion.

[Fordeatinz71]

yea, i was gonna say, if they are vented i've got a big issue...my truck lets out a healthy WOOOOSH of air every time i remove the cap...although i do it slowly, lol.  first time i ever had a carb'd vehicle that did that...

[ccz145a]

My '75 has a third outlet on each tank sending unit with a 3in hose stub and the same vent fitting as you see on the rear differential hose. It's factory. One of the last pre-emmisions half tons

I took it down to the frame last July in sunny 100deg weather and by the time I got the frame and tanks derusted and painted (black) and the cab back on and ready to crank in January, all my gas had evaporated 

I know it had at least 5 gal. in there when I started... Guess black wasn't the right color to paint the gas tanks!

Point is: even when the fuel is vented, it's not at the cap.

[Stewart G Griffin]

Update:

Apparently Nissan sentra caps fit 83 fullsize trucks.