Not so Smelly Austin

[andrew34ruby]

My Ruby tank breather is not blocked and I get a strong smell in the garage sometimes, but not all the time. It depends what brand of fuel is in the tank and the smell lasts for maybe a week after filling the tank. When the smell is at it's worst it really is strong, but at the moment no smell at all.

[Ilmoro]

Those of you who have blocked the breather....Is it more difficult to fill the tank due to backpressure?

My overflow is currently open and I only get a petrol smell when I have (over) filled the tank and car movement causes some petrol to come out of the breather.

Ian

[AustinWood]

Petrol is very volatile. If you can smell it and especially if there is a damp patch, then probably quite a lot is being lost.

[Parazine]

Those of you who have blocked the breather....Is it more difficult to fill the tank due to backpressure?

No, why would there be backpressure? It's not a modern system, sealed up everywhere. Petrol goes in the filler neck and air comes out of the same place, no hidden bits. The pump nozzle doesn't fill the filler neck so plenty of room for the air to escape.....

[Ilmoro]

The RM riley has back pressure and that isn't a modern car either....Just wondering why Austin went to the trouble and expense of providing a breather tube?

[Bert]

Early front tanks don't have such problems.
Bert.

[A G Wood]

The RM riley has back pressure and that isn't a modern car either....Just wondering why Austin went to the trouble and expense of providing a breather tube?

Speaking generally, any car with a sealed fuel cap will have to have some other method of fuel tank venting to avoid collapse of the tank due to the partial vacuum being formed by the fuel being drawn out of it if it is a pumped system [the humble camshaft driven mechanical pump can produce around 10" Hg 'vacuum'], or to avoid fuel stoppage in the case of gravity feed. The results of a blocked vent or the fitting of a non-vented cap where a vented one is indicated can be quite spectacular on pumped systems...  When you say 'back pressure' on the RM, do you mean there is a pressure release you can hear when you remove the cap, or do you mean excessive splash back when you try to refuel? If its the latter, then apart from an excess of 90deg bends in the system, I have seen non-fuel rated hose in the filler plumbing swell up internally, sometimes down to as little 15-20mm from a nominal 50mm ID, which naturally tends to severely restrict fuel going in as well as air escaping out during the filling process.

[Ilmoro]

What I mean is, the petrol going in, especially from a modern forecourt pump, causes petrol fumes and air to come out of the tank at a velocity capable of levitating the ingoing petrol out of the filler pipe. The RM petrol tank and filler commonly cause this to happen if you do not pump the petrol slowly. Indeed, the RM rileys have two filler caps for the single tank, one on each side of the car and Nuffield (whispered in current forum) acknowledged the issue with back pressure by the following instruction in the vehicle handbook. (see attached) Incidentally, there is no problem with sucking a vacuum in normal operation. My original question was "Has anyone experienced issues with backpressure when filling an A7 tank (with breather) I assumed there was sufficient air leakage around the filler cap for tank vacuum not to be a problem. As no-one has come forward to state there is a problem...I assume there isn't. So why did Austin fit the breather?

[Parazine]

So why did Austin fit the breather?

Dunno. As stated, earlier cars didn't have them, later cars don't need them.

[Colin Morgan]

Found this on the Hereford A7 site from 2017:

"Rear fuel tank breather
From August 1934, the low chassis Austin Seven Jewel range were introduced. From the
introduction of these new models, all petrol tanks were in a concealed position at the rear of the car
with a steel filler pipe projecting towards the rear wing. The actual filler pipe was joined to the tank
pipe by a flexible hose which was quite close to the spare-wheel compartment.
Austin realised that any leak from the filler pipe caused by overfilling could result in petrol fumes
entering the car. To prevent this happening, an overflow pipe was fitted to the front of the petrol
tank just behind the back axle. So, if petrol is seen dribbling from underneath the rear of the car
when filling with petrol, you have probably overfilled the tank. This is not an uncommon occurrence
especially as our elderly petrol gauges become less reliable. To prevent this happening, it is possible
to plug-off the overflow pipe either with a tapered hardwood plug or with a central heating
microbore (probably 10 mm) blank-end. (slightly less safe to smoke in the car though …. Ed)
Clearly, it is best to avoid over-filling ……. Eddie Loader"


Also, there was a discussion on the previous version of this site, many years ago, concerning the expansion of petrol after filling up from a cold underground tank - with the possibility of overflow then occurring should a tank be filled to the top.  Sandy Croall calculated that expansion might be as much as a pint on a hot day.  This could be enough to set the overflow syphon effect off?  However, if the the overflow is blocked, then any expansion would occur (relatively - assuming no leak) harmlessly up the filler tube, with air escaping from the unsealed filler cap?