Hi and welcome to FixYa,
The description of being in storage is significant as it indicates that the carbs require a pulldown and disassembly; soaking in carb cleaner, cleaning the jets, vents, holes, ports and the walls of the bowl and inner body for the varnish like gunk of old/stale gasoline as perhaps you have done already.
On switching on, the fuel pump is activated and feeds fuel to the carbs. The floats in the carb's bowl would then rise up and engage the inlet cutoff valve closing the inlet port and therefore no further fuel would flow in even if the pump is still working. Fuel overflow would then indicate possibly that:
- the valves are not closing;
- their is an obstruction along the inlet ports and blocks closing of the valves;
- floats are stuck open;
- the float height would require adjustments;
- necking, ridges, grooves, scratches exist on either the valve or the port allowing fuel entry even if closed.
All of the above would require a pulldown of the carbs for inspection, further cleaning or applicable repairs/rebuilt.
Good luck and thank you for asking FixYa.
