Harv. Just throwing out an idea for a wierd problem. I have no idea what the effect would be.

when you do confirm some vapor is coming out whatever location, that will confirm what you have observed. But it should not be there.

Be sure to follow the advice of Cupcake and Mur -seems like it should lead you to the culprit

Answers to the above questions:

a) Nope, external fuel pump, mounted in the battery box

b) Not sure of the question, but it's a simple impedence-based in-tank fuel level sensor - circa 1973

c) tank is mounted securely and does not move in reference to the body

d) This is a rust free shell with no rot or open holes in the boot floor.

e) I have not confirmed this since I posted this past weekend.  I will check that.  I'm thinking putting a rubber glove on the fuel filler neck and vent line to see the rate of flow out of both outlets.

In response to zippy's comment - the fuel in the tank, the tank, the fuel added to the tank, and the surrounding air were at at the same temperature.  I filled the tank in my garage using a 5gal fuel jug that was also in the garage.  It seems reasonable that the difference in temps between the old fuel and new fuel could cause that, but not in this case.

I will reply again once I gather more data.

I think this perceived air pressure is a result of the facet pump hammering away in an uninterrupted fashion. But by all means, fit a barometer of some sort to the vent pipe and find out how much pressure there is in the tank.

Cup, does this mean the "air bubbles" are actually 100%  fuel vapor pulled by negative pressure out of the liquid? 

My ideas (other than the vent) were zebras - in the category of "when you hear hoofbeats, do not think of zebras"

the only experience I had with bubbles was 30 years ago when we had to drain, flush, and vent volatile gasoline vapor out of diesel fuel systems (because the renters did not heed the "diesel fuel-only" placards posted everywhere.

The inlet/outlet pipes of the fuel pump are oriented wrongly. The outlet is supposed to be down and the inlet up.

I think Mr. Lankford is correct.
If the vent system is working, you won't build any pressure in the tank.
When you fill the tank the fuel is pretty darn cold*. As it heats up to ambient temp, it'll expand and cause pressure.
Do you have the room to be able to blow through the vent hose with the fuel cap off? If not, figure out some way of extending it. With the fuel cap removed, you shouldn't get much resistance to your lung power.

*This is why you should try and fill your tank in the morning. The fuel will be slightly more dense giving a small amount more than warmer fuel...
On some cars, if you fill up all the way up the neck when cold, it could expand enough to dump fuel as it warms.

first - airbubbles/fuel bubbles/dangerous bubbles going backwards theory?????????

here are some WILD AND CRAZY ideas since no one else has solved the problem yet:

(A) is the fuel pump a modern submerged-in-the-tank variety (and overheating and vaporizing fuel? Boeing had that problem).

(B) is the fuel level sensor ...same issue? Never heard of that happening, but remotely possible.

(C) is the tank not strapped down and therefore bouncing around, sloshing and evaporating fuel?

(D) do you have a big hole in your boot floor, with exhaust heat rising up and heating the tank? gulp.

(E) MOST IMPORTANT: if the vent tube really is venting, then you should have no pressure build-up from whatever reason. You may have checked the vent rubber hose itself, but what about the metal nipple it slides on to? Is it clogged or corroded? If you test this hypothesis, don't use a metal (spark-risk) wire to ream it out, use a thin plastic something. Even better, and easier-to-get-to,  just use a source of mild air pressure at the end of the hose under the car to see if you can punp a small whiff of air backwards through the installed hose AND the nipple into the tank. (Tank cap should be off when doing this). I have never done this, but it is an idea to consider.

Mur --

It's a 1973 car, so it doesn't have any emissions equipment like a charcoal canister.  There is a single small vent tube (think vacuum hose) exiting the top of the tank.  The vent tube is not clogged.

How could crank case pressure get from the engine to the fuel tank when the only connections between the fuel tank and the engine are wiring harness and fuel feed line??

Since this Mini is taken to the race track from time to time a mechanical fuel pump is not an option.  I only run 91 or higher Chevron fuel in it.

Does the car have the evaporative emissions system, and is it complete? The tank has a vent at the top and the vent tubing is cheap plastic and failure prone, while the line up the right hand side of the car to the charcoal canister is likely to trap fluids and rust or clog or both. 

I suppose crank case pressure going back via this system could create your issue.

I would run a stock mechanical fuel pump on that car. I know there are a lot of opinions available for free on the Internet, and my own tends strongly towards simple and reliable solutions. Newly made pumps seem to be OK with the hideous crap in modern fuels, especially in the U.S. Midwest. I do know that the only facet pump I ever ran caused issues: blown head gaskets from flooding carbs when the fuel pressure regulator failed, stalling at speed because the fuel pressure regulator re adjusted itself.

in any case, the fuel pump is not your present problem.

I really can't see how this is possable?? if there is no return to the tank how would it get a + pressure? even if the pump was "backing up" because it's being strangled down to 3psi I still fail to see it pressurizing the tank?

He all --

I have some strange behavior that I could use some guidance on.  I was warming up my Mini yesterday and decided to top off the fuel tank while the engine was warming up.  To my surprise when I removed the filler cap I noticed that there is positive pressure inside the tank.  If I cup my hand over the opening with a 1/2" gap between my palm and the filler throat I can easily feel air flow exiting the tank (similar in feel to putting a hand in front of the exhaust but not anywhere near that pressure).

The pressure is only there when the engine is running.

Vehicle information

• 1973 Canadian LHD mini (canadian is important because they use different fuel tank necks and caps than British and US import minis)
• 1275 A-series block
• Weber 45DCOE carb
• Facet silver electric fuel pump
• Holley adjustable fuel pressure regulator set at around 3 psi
• No fuel return line from carb

A friend suggested I look for bubbles in the tank to see where the air is coming from but it's rather difficult to see with the fuel sloshing around when the engine is running.  It seems reasonable that the bubbles would be coming the fuel outlet filter, but why?

Anyone have an idea what would cause this symptom?

Thanks in advance for anyone that offers helpful information