Derek
Apologies for an omission in my last message I should have been clearer that the needle I referred to in the last sentence is the needle jet needle. also that I had the same problem with leakage.
The two washers on top of the float make it heavier, which means the petrol level will need to be higher to lift the float. You should start by taking the washers off the float but also make sure the float is very clean, and see if that cures the problem.
I worked out the level in the float chamber by floating the float and needle assembly in petrol, in a suitable glass jar. Then noting the petrol level on the float and marking it. With the float and needle installed in the float chamber top, in the valve closed position, I measured from the mark on the float to the flange on the float chamber top. This measurement transferred to the float chamber/ carb body gives the petrol level. You can then confirm that this level is correct or make some adjustments to get it right.
The other thing worth noting is that the SG of petrol in the 1930's was probably higher than petrol today (0.72 - 0 .775 for BP Unleaded petrol which includes the ethanol) I would guess that, despite the ethanol, modern fuel has more light ends as the refining process is much more controlled, however the principle of the float having to displace its own weight to float hasn't altered. That means today the petrol will probably be higher up the float before it lifts it and closes the valve making the possibility of leaking through the main jet higher..