I was getting ready to go the AMES Pontiac Nationals and going over car. Noticed a fuel leak near the fuel pump. It was the 1/4" return line. I replaced the small hose between the steel line and pump. The line was under a large amount of pressure, blew gas everywhere, possibly contributing to the leak point? Later at each gas stop, the gas cap would nearly blow out of my hand, hmmm? The leak was fixed with the new hose but it doesn't seem right to me. I don't know how the 67' 400 fuel system is supposed to vent??
I have a 69, but my tank has a vent tube welded in the neck of the tank. On this was a hose that vents thru my rear tail panel in between the bumper and is not visible to the casual viewer.
There are also vented caps that you can buy. I would get a vented cap and see if your tank is vented. Again, not sure about 67's but if not vent in the tank itself you most likely need a vented cap.
67 firebirds had a vented tank cap, not non-vented didn't know they had a fuel return line on 67's. It sounds like your tank is building up pressure, if you can't find a cap, or clean out the check valve in the cap I would leave it loose or it'll blow out the hose that connects to the tank to filler neck. I went looking for a cap earlier They don't seem to make one at the moment. NOS would be the way to go.
Most of the aftermarket "vented" caps only vent one way and do not vent excess positive pressure. They only vent when there is negative pressure (the fuel pump drawing fuel). If you take it out of the cool garage and go out when its hot it will still have pressure on it when you open the cap. This was responsible for blowing out my sending unit gasket.
Now I run a vented cap with a 1/16" hole drilled in it (as Bjorn so wisely showed us). I couldn't find a vented cap that vents both positive and negative pressure. A '9 has an open vent right in the filler tube and vents up by the trunk with a small hose. I figure I'm doing the same thing, but could possibly spill fuel on a hard launch (which I almost never do. LOL !!) Haven't spilled any yet that I know of.
I cannot comment on actual stock '68 FGF vented caps because I can't find one.
I just checked the cap on my 68 and it vents in and out, I did not buy this car from new so it could have been replaced. I also do not have a return line. Does the return line just return vapours or does it also return excess fuel the carb doesn't use?
Can I ask how did you check to determine it vents both ways? I've had a hell of a time finding one that fits that vents both ways. Any chance you have a picture?? I'd rather have one of those.
And IDK about the return line as I do not have one. There was a guy on the google group list had an issue with his almost new pump that was NFG and returning most of his fuel back to the tank instead of the carb(y) when it got hot the car would die. I haven't had any vapor locking issues yet without a return line.
The cap has four holes near the center under the top. I sealed the cap around the gasket and applied pressure and vacuum with a pump and had venting both ways. I'll try to get a pic up later.
I owe a huge thanx to Craig (68tpls400) who emailed me off list a pic of his original cap. Mine which I thought may be original clearly is not. He inspired me to tear mine apart, I discovered a little check valve disc and a rubber seat. I put 2 notches in the rubber seat and now it vents both ways.
Wish I'd taken it apart 2 yrs ago when I drilled out a brand new one.
