Morris Minor Owners Club

Hi, as my MM1000 lives under a carport on my drive I fitted a lockable petrol cap for security, but now I have a problem with the tank pressurising, could somebody tell me how to fix this problem.
Thanks, Bernard.

You need ideally a locking cap with a breather to allow air into the tank as it empties. Not all readily available caps have them. The breather is often a VERY small hole drilled through the top surface of the cap, close to the outer edge.

If the tank is pressurising i.e. the air not being able to get out conversely the air that needs to get in to replace the volume of petrol pumped to the engine cannot get in which will make your petrol pump struggle.
If there is no vent the expansion of petrol in the tank will pressurise the tank.
Dependent upon the type and make of lockable cap fitted a hole drilled in the underside of the cap would alleviate the problem of tank pressurising.

Not all vented caps have an obvious hole - I have a lockable vented cap which has a "labyrinth " cast into the underside which provides the ventilation, so it's worth looking at a cap closely before assuming it isn't vented.

If the lockable petrol tank cap was vented there would not be a problem with petrol tank pressurisation.

Check that any vent hole (if there is one) in the cap is not blocked.

I've heard of petrol tanks being crushed from atmospheric pressure due to the vacuum caused by the pumping out of its contents!

Alan

That is one of the few thing I remember from by BL / Unipart product training back in 1979