Rezlo - I'm very sorry to hear of the trials you've been put through. Without knowing more, it's indeed possible there's a crack in them, or a casting imperfection, and when 'warm' the leak is not visible because the fuel can evaporate at the same rate it's seeping out a small hole.

For what it's worth, sometimes there is good sense in the peace of mind of starting fresh with a known new and flawless carb.

In very very general terms - I'm not going to make out I'm the world expert on carbs, but I've done a fair bloody chunk of time on a bunch of different carbs - webers as a general rule should _not_ go out of tune. Almost always (and this goes for SU type carbs too) if they are going out of tune, it's actually traced back to stuffed throttle shafts, which never re-set to the same exact amt of air flow when they go back to the idle position, as they are leaking air through hte shaft, and/or it's a case of weak/flimsy linkages which likewise don't produce consistent/equal opening from carb to carb.

If there is one achilles heel of webers, it's already been mentioned - they are EXTREMELY sensitive to fuel pressure, and anything above around 3-3.5psi fuel pressure and they will over run the needle and seat, and produce inconsistent and overfuelling because they are overfilling/flooding the fuel bowls and then the engine. EVen many std factory mechanical fuel pumps have an output too high in pressure to be appropriate for a weber. If it's only leaking overnight, maybe just maybe there is a blocked breather and the tank is ballooning from petrol vapourisation over time, but that's a long shot imho.

For all you know the reason these sets of 40dcoes came up for sale was there was a previous issue with them, and somebody has sold them on and kept quiet about the known problem, leaving you to do a sherlock holmes and try and find out why.

What else comes to mind - actually here's an odd tid-bit. I've mentioned here and there that I've been into early (i.e. the original ones) minis, and also the locally produced hemi 6 cylinder chargers. An odd thing I can mention is that both of these have been weber shod (the hemi 6 pack chargers had triple webers from the factory, and any historic mini racer runs a single weber on a long neck inlet, or possibly twin split webers). Now here is the funny thing. The hemi guys, as a rule tend to run a specific 'solid' style gasket between the weber and the manifold, and swear by them, as the rubber ones allow too much vibration (this is partly because they are on a big tall 6 cylinder, with a moderately long intake manifold, so they are spaced out fairly far so 1-2 degrees engine movement = a greater distance of travel up that high and out that far to the side. Now on the other hand, on minis, these solid gaskets see the carbs with short distance but high frequency vibrations, and the needle and seat can't keep closed, and they overfill and are all over the place as far as consistency goes. Nearly everyone in contrast on a mini runs the rubber o-ring imbedded gaskets between weber and manifold and is sure not to overtighten the nuts holding the weber to the manifold - and on the mini that combo tends to dampen out the vibrations and keep it 'on song'. I am aware of an exception or two here or there, but that's almost universally the different case for each engine. it's a long shot, but maybe give the opposite to whatever you are running a try and see if it worked.

Having said all that, I'm sure everyone here (myself included) wishes you a 100% trouble free upgrade, and the end of all hassles, with the fitment of the 45dcoes!!

About the only other thing I can think of with the old 40s is to maybe add some type of really strong/thin dye to a load of petrol, and run the fuel pump to fill the fuel bowls, and then leave overnight and see what happens - maybe the source/location of the seepage would become apparent??

Good luck mate!