Having fixed my share of carburettor problems on my MG V8 (SU HIF6) recently, here are my suggestions:

1 - Check the float needle (part 156). I have had those sticking on multiple occasions, especially if the car has been sitting for a while. Clean them out with royal doses of carb cleaner. Air won't do it. Carb cleaner (and fuel) need to squirt out massively. I had to clean a needle twice the other day before my SU carb worked again (and the needle was NOS!).

2 - Check whether your float (and its shaft) are sticky. Normally, fuel pressure from pump will blow it open, but if float can "float" over its shaft, it may become sticky. Very rare, but it can happen.

3 - Make sure your float level is adjusted correctly as Tom said. In general, the workshop manuals seem to make a bigger deal about the precision than is necessary (in my opinion). The bottomline is that you want to make sure it closes, but at the same time it should allow enough fuel in the bowl. I have had to adjust the floats of one of my previous MGs also because it seemed to starve a bit.

4 - Vacuum/"false air" leak would be my next chase, although if you have new gaskets, etc, I can only think about having forgotten to close off some orifice in the carb or so as you problem. Check what the starvation symptoms are under different load conditions. It may give you a clue as to whether it consistently runs out of gas after a certain time (float filling problem), or whether a sudden increase in load causes the engine to starve. The latter is more typical of an air leak (or a clogged carb jet).

If possible, try to check air flow/tuning on each carburettor to see which one is the problem case. I had 3 weeks ago an interesting problem (when trying to get my MGB V8 going again after sitting for a while) that adjusting one of my SUs never changed the engine speed (!). Whatever I did to the set screw, nothing changed. Turned out the needle seat was stuck, followed by sticky float needle, followed by yet another (NOS) float needle that was sticky, followed by succes which is a incredible smooth humming 3.5 liter V8 with no blue fumes or barking (starving) exhaust anymore. But it did take 3 disassemblies of the carburettor. If both carbs act the same way, poorly working fuel pump would be my next bet. It may reach pressure for a while, but may not be able to sustain it and necessary flow rates.

And change the fuel filter in case it is old. And I have seen one case where the lining in a rubber fuel hose started separating and clogging (like in MB brake hoses), so if they are old, just put some new ones on (for safety too).

Good luck,

Bert

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'70 111 280SE/c 3.5 (4 spd manual) - sold
'63 MGB
'73 MGBGT V8