5mm is enough. Rebuild kits for Weber downdraft carbs should be available locally. It won’t hurt to change that needle valve but your problem is present when the engine is switched off, the fuel pump has stopped and pressure in the fuel line will be steady or falling. The percolation of fuel, as Toby has hinted at is due to it expanding more under heat than old fashioned blends. The phenolic spacer will help this but you could also tweak your float height a millimetre or two lower to reduce the spill over level in the carb somewhat. Watch out that you don’t create another problem with your tune though.