Hi Tom, I've seen the 'hot/cold' issue before, although with individual cylinders, not the entire bank. At least on my setup, minor adjustments to the idle mixture screw at the base of the carb can bring a cylinder with 'cold' exhaust to idle properly. (There are of course eight - one per cyl - and none end up in the exact same position; I must dial-in each cyl individually to give maximum revs at idle at a given throttle setting. As they get dialed in, engine idle rises, so the throttle setting for all carbs must be reduced collectively to a normal idle speed.
I use a homemade extender to reach the screws (pic 2) - small nail in an aluminum tube - on my obscured LH carbs. RH is easy w/ this side-pull 'reversed' setup. On your car clearly no adjustment screws are easy to reach
If your carbs are not linked together that complicates the above idle adjustment...I guess you could throttle an individual carb back down once 'best' idle mixture settings are found, but then the sync setting is messed up again. I would connect the linkage, sync all carbs (ideally with a setup like Rene's), THEN adjust idle mixture.
The only other thing that came to mind was fuel pressure. I know everyone says you need c. 3 psi for Webers but I have accidently run 8-10 at times due to a hair-trigger Filter King pressure limiter, and the engine doesn't seem to care much(!). So maybe it's not a HUGE deal but surely best to run c. 3-3.5 psi.
Please let me know if you know of a Weber expert in our area, I'd like to have my system evaluated at some point, once everything else on the car is good to go....