I messed with mine for 3 years and never got it to fire quickly after sitting a week. Always had to use starting fluid. Tried all the fixes above, none worked. No leaks - mine at least was evaporating.

Did a mild warm up of the engine and changed to a holley 850. Now, It can sit for well over a month and fire easily. Pump throttle once, turn key and it fires within a second or two.

IME, the webers just allow evaporation of the fuel.

The old carter on my Mopar does the same thing. Evaporates out of the bowl. I use a mustard bottle to fill the bowls before cranking.
I'd refrain from using starting fluid if possible.

Would a phenolic carb spacer help?

Makes a huge difference in heat to the carb since it is the super heated intake manifold (OE carb intakes have open exhaust crossovers in bottom of intake - not the Vortec sbc's though) that causes most of the heat.

2-3hrs after the engine is shut down....no difference.

Mine does it too. It cranks for about 5 to 8 seconds and starts cold. Electric pump. I may wire a prime button.

Same problem when I had the Weber on the SBC in my bowrider. When I did some work to the motor I swapped to a Holley and never had an issue again. My Regal with Weber's on 454's does the same thing.

Had the same issue with a Weber on a 454mag. I swapped to a Holley when I did some upgrades.