Hey guys & girls,
I have a 1.5 su, it has 2 inlets on the float chamber, obviously one is for fuel line question is where should the 2nd one be connected, reason im asking is when I took it for a drive it was breaking down,im wondering if the 2nd one should be connected somewhere as it may be sucking too much air?

Originally it may have been connected to a charcoal canister as part of the evaporative loss control system. However the issue you are describing would not be caused by the float lid venting to atmosphere.

Some have feed and vent/overflow ports. Make sure they aren’t the wrong way around, it’s also a good idea to direct the vent/overflow port away from the exhaust manifold

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Thanks guys, I'll have to have a look at it tomorrow and see whats going on

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.

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1963 Morris 850

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.