Fuel tank pressurisation in summer.

[seadogwoody]

New to club but had morris minors before, thought ain't no expert.
My 1971 traveller has been getting the fuel tank pressurised with the heat of the summer, so I've noticed a little fuel leaking now & then after a trip from the flexible pipe to the carb & even from the screws that hold down the float chamber lid.
I've tightened everything as much as possible with out breakage, but still get this sometimes. The fuel filler cap when agitated let's out the pressure with a his however I would think there's a vent somewhere....any pointers please!
The whole thing seems to be on the dangerous end of comfortable to live with, so is this a known issue, it's never happened with the previous morris.
Thanks in advance.

[philthehill]

The factory fitted fuel cap is vented so there should be no pressure being generated in the fuel tank by the expansion of the fuel due to warm weather.
That begs the question - what filler cap is fitted to the petrol tank?
A photo of the filler cap and filler pipe would be appreciated.

[Bill_qaz]

Your fuel cap should be vented, perhaps it is blocked?
Simultaneously posted with Phil

[exlkrs]

Have you a locking fuel cap or original? Possibly a locking one may need a little hole drilling in the back plate.

[geoberni]

The one question in this is how is it pressurising in the Summer Heat, but not causing issues the rest of the time as the pump tries to suck against a vacuum forming in the tank?
Either it's a vented cap or it isn't.... They aren't intended to be unidirectional.
Perhaps the vent hole in the cap has only recently become blocked.
Very strange.

[seadogwoody]

Thanks everyone for Ur reply & advise.
Have checked the filler cap which is the locking type, & there's no vent hole to be seen ... so now there is & as the day has warmed the pressure has not returned
Ur right though 'geoberni' how did the pump suck against the vacuum
Or even the pressure get past the pump
Well at least that's not going to continue worrying me, thank you all.

[myoldjalopy]

I hope this is fixed. Now take the car for a drive and see if there are still leaks from the pipe to the carb and the float lid screws.

[philthehill]

'How did the pump suck against the vacuum' - if there is pressure in the tank (not a vacuum) the fuel is pushed to the pump and the pump inlet valve is held open and will let fuel through. There is no spring under the valve to close it, it just floats and moves according to where the pump diaphragm is. The pressure required to hold the valve open in minimal.

The line pressure to the carb is around 3lb

'Or even the pressure get past the pump' - the pump outlet valve is just a flat plate and allows fuel to flow one way towards the carb, as above again there is no spring it just floats according to where the diaphragm is it will allow fuel to exit the pump.
So to cap - if there is pressure in the tank - the pump inlet valve is held open - the outlet valve is pushed open by the pressure and fuel flows to the carb without assistance from the fuel pump.

[olonas]

I have a lockable fuel filler cap fitted. There's no obviously visible ventilation. However, on examination, there looks to be an aperture above the sealing disc and rubber ring through which it possibly vents out of the keyhole.
i.e. when the cap is in place, pushed down against an internal spring and turned to secure it in position, a steel disc with a rubber washer attached seals the opening but exposes that small aperture to atmosphere (via the keyhole).
The temperature in my garage, at the moment, is 27 degrees centipede and there's no tank pressure release when the cap is removed.

[StillGotMy1stCar]

The temperature in my garage, at the moment, is 27 degrees centipede and there's no tank pressure release when the cap is removed.

I’m glad I’ve seen this in Black-and-White, Centipede is better than Centigrade or Fahrenheit

[olonas]

And the Fartingheight temperature scale is far more accurate than Fahrenheit.

[seadogwoody]

Well as a 'Newbie' I'd like to push this chat a little further ... If the pressure in the tank could push past the fuel pump & pressurise the float chamber, then could it have an influence on the mixture in to the engine hence making it run rich?
The line from pump runs at 3lb so an increase of 1or 2lbs may have an effect but minimal that the car runs not well but not that badly, & only when the pressure is higher?
Interesting thoughts?

Shall take for a run after clutch adjustment, so will check on leaks etc too.

[philthehill]

Any increase in fuel line pressure will not pressurise the float chamber as the float chamber is vented.
What it will do is increase the pressure on the float valve and if excessively high will lead to fuel leaking past the float valve and increasing the level of fuel in the float chamber so increasing the fuel mixture.

[jaekl]

I believe the concern about the pump dealing with a vacuum is when the tank was not exposed to warm temperatures with an unvented tank.

[seadogwoody]

Thanks, so that could give reason to very advanced timeing on the ignition, as it was when I brought the car? The timing was set by sound of the running (I think) rather then strobe & carb mixture adjustment as I did.
Thank you for Ur help, & that goes for all.

[geoberni]

'How did the pump suck against the vacuum' - if there is pressure in the tank (not a vacuum) the fuel is pushed to the pump and the pump inlet valve is held open and will let fuel through. There is no spring under the valve to close it, it just floats and moves according to where the pump diaphragm is. The pressure required to hold the valve open in minimal.

My comment was nothing to do with the 'pressurisation in the heat of summer', it was the other 90% of the year, when the tank is cold and the fuel level drops, then a blocked vent results in a vacuum for the pump to fight against.

How many times have there been posts here about people having fuel starvation problems and the recommendations include 'check the Cap is vented'...??

[seadogwoody]

It's been enlightening & pleasing to have brought this subject to post & received such great responses thanks all.
I ain't had the opportunity to really road test after the adjustments, but she starts & runs well, & hasn't leaked or hissed throughout the day, so in my book that's success.
Once more thanks, Dan.