mini0998 wrote:
Maybe check first as I have a SU here that has the vent hose fitting there but isn't actually drilled through. It has the small vent hole in float top housing instead.
There are a lot of SU float bowl lids with 2 or more inlet/outlets - to feed a twin carburettor setup or tank return.
If your second outlet was drilled through, it won't be sucking in air but petrol would be coming out of it from the main feed line.
Photo of a lid with 1 inlet, showing the pressed metal baffle covering the small overflow breather hole.
Your "breaking down" might be a partial blockage in the fuel supply, letting the bowl fill up slowly and drop low when driving.