I have an 1984 with a 300 inline 6. The intake manifold had two cracks in it so I decided to go ahead and do the Offenhauser intake with headers and a Holley 4 barrel. I also decided to replace the gas tank. Because of all this, I'm deleting all the smog related stuff and doing new exhaust. On top of the gas tank is a white plastic valve that hooks up to a line that goes up to the engine bay. I believe this to be a fuel vapor return line. My question is, do I even need this line anymore? Was it just part of the smog system? Can I just cap it off at the fuel tank? I guess that's really more than one question. I currently have the bed off the truck while I'm replacing the fuel tank, rear shocks and some other stuff. I'd like to figure out this vapor line thing before I put the bed back on. Thanks!

 

A whole bunch of whys come to mind
A 4bbl is too much carb / fuel for a '84 300-6
That is, unless you are going racing and install higher compression pistons and an aftermarket camshaft
More whys on deleting the smog
We all breathe the same air
Of course, you can delete the vapor vent, but it might be hell to fill the tank
That vapor vent is supposed to go forward up to the charcoal canister
You got something against charcoal canisters and burning vapor like it is supposed to do?
IMO
You'll be better off with a 2 barrel adapter on that 4bbl Offy intake and leaving ALL the smog in place
Granted, the factory Carter / Motorcraft YF-1A carb leaves a lot to be desired
Cutting down some of the EGR flow helps immensely

 

4 barrel on a 300?!

 

Thanks for explaining the vapor line routing. I have nothing against using the charcoal canister, just wanted a little clarification on the what, how and why. As for the 4bbl, it seems to be a fairly common approach, but I'm not wed to the idea. Here are a couple of builds I found interesting: