[Gaz64]

We need to work out whether the fuel is leaking internally or evaporating (or both).

If you cold start the engine and run for a short period without putting a lot of heat into the engine, then let it sit for 2 days, do the float levels stay correct or drop down as stated?

If they do drop, I'd look at the power valve gaskets and/or the power valve diaphragms for leaks.

If they don't drop, then I'd be looking at adding a phenolic or wood carburettor spacer to help insulate the carb from the engine temperature at hot shutdown.