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I have problems starting a hot engine or sluggishness if crawling at low speed (75 3.0litre QV) so I have been “experimenting” with the fuel system. There is a small-bore vent pipe from plastic breather unit(?) on top the fuel tank to the inlet manifold via the carbon canister, which is controlled by a small solenoid – switched on with the engine running.
Now, I have by-passed the carbon canister and I have some interesting results; when removing the fuel cap there is a lot more pressure, so much so that you can feel the pressure against the cap. The engine appears to be stumbling less and idling better but I can hear the fuel tanking “buckling” on start up – is this normal?

Update on this - it was not pressure acting on the petrol cap but vacuum trying to keep it (the cap) & hence the buckling sound!
So why is it doing this? This means the tank is not breathing.
There are two check valves attached to the "breather tank", an outlet that goes to the inlet manifold via the carbon canister and an inlet, which should let it breathe. Testing this by blowing through revealed that although it was functioning OK, quite a lot of effort was required; this has now been replaced with a spare.
Well, I have been driving it for 2 weeks in very hot weather with little fuel in the tank and the car has never felt better.