Good spot! Looks like some sort of pressurised bottle. Surely can't be nitrous could it? If it is pressurised, then it is pressurising the float chamber. those are supposed to be overflow pipes that should have attached to the back of the original airbox. A pressurised float chamber will either cut the fuel or as I am probably thinking, it will force it into the intake and stop the float from working as it should thus flooding the carb. Float levels are really important on SU carbs. I would disconnect those to start with.Not an expert with carbs, but just a question for my edification. The radiator overflow and what appears to be tubes from the carb float chambers are feeding into the same blue bottle/tank like thing. I’ve not seen anything like that before, whats the purpose of that? I can’t see that being very beneficial to the operation of the carbs, are you sure that’s not the cause for your problems?
I've said this so many times - make sure your fuel filter isn't blocked. That's where I'd start, it's a common problem with these cars.Fuel filter top left of screen looks rather dark is there rust in it. From tank.