A pal sent me a video of experiments with a plastic carb that shows what's happening inside, using a high-speed camera. Rather long at 25 minutes and with too much OTT chat for me, but the high-speed footage is very interesting and well worth a view. The full video is here, but I have extracted the high-speed parts (and one section of chat which will be mentioned further on) lasting just over 3 minutes:
I did try to download the video for editing, which I've done before, but it didn't want to do it this time, probably because systems out there keep changing what they will work with on my computer, and that's probably because it is rather old being Windows XP. So had to resort to filming my monitor.
Starting with a slow, then a very slow, view of the whole carburettor, and fuel vapour on the engine side of the throttle butterfly at each intake stroke.
From 30 secs you can see the end of the intake stroke puffs a bit of vapour out back towards the intake, probably from the way the intake valve doesn't close until after the start of the compression stroke (56 degrees after in the case of the MGB). At 1 min you can see a similar effect from the smaller overlap with the exhaust at the beginning of the intake stroke (16 degrees in the MGB). Particularly interesting is how fuel continues to rise from the jet after the intake stroke has ended, probably from the inertia of the fuel, and just hangs there in the venturi until the next intake stroke, instead of having to pull 'fresh' fuel up from the jet. How much is there, and how long it stays there, will be a function of the rpm of the engine.
From 1 min when the stream and droplets rise at the end of the intake vacuum you can see liquid fuel running down the slope of the engine side of the venturi. Clearer still from the different angle at 1 min 20. It seems to me that this effect is what causes HS carbs to bog down when left idling, and a brief rev is needed to 'clear its throat'. My HIFs don't do that, probably because they have a 'sump' immediately after the jet and a passage from there to right by the butterfly, which will clear this excess fuel away.
At 1 min 44 Dad says he would make the controls stiffer because the engine sucks the throttle fully open, which I have my doubts about. You can see earlier that the butterfly is unrestrained, and it does not fly open. With as much butterfly surface area above the spindle as below there is as much suction on one half of the butterfly trying to close it as there is on the other side trying to open it. At 2 mins 10 secs you can see the intake stroke sucking both halves of the butterfly towards the engine, because heat from the lamp used for filming has softened it.
At 2 min 45 the engine is being started with the pull cord (which is in the way a bit) but you can see it takes a couple of intake strokes before any fuel has got up the jet into the venturi.
Just after that he stalls the engine and you can see how the intake remains full of fuel vapour when the engine has stopped. And although one of them says he didn't see vapour come out of the intake side of the carb it is there as he removes his hand, and remains between the butterfly and the engine for some time.
Finally at 3 min the camera is focused into the intake, with the choke butterfly having been removed, and you can see how the hanging droplet of fuel seems to broaden out before being vaporised, the vapour filling the whole of the (rectangular in this case) throat.