Here's my two cents worth. I can't remember the differences between the float chamber set-up on Stromberg's for the US market and those for other markets. Suffice it to say, there were several major ones - and all to do with the emission control system. As we were never allowed to strip a US spec Stromberg, I never got around to finding out what those differences were. My own GT6 suffered fuel leakage and this was coming from the float chamber. It was a common problem (subject to service bulletins more than once) and it affected SU's as well.
The delrin needle (on an SU) and whatever's in a US Stromberg allowing fuel to enter the chamber is prone to sticking. In fact, you can't see the 'stiction areas' with the naked eye or even under a magnifying glass. The longitudinal fins on the SU needle are/were the culprits as were the GT6 valves. Service bulletins advised a valve change - but that was just to sell more of them!
Take the valve out and clean all its surfaces as best you can (ESPECIALLY the fins) with a piece of paint flatting paper around 300 or 600 grit. If you don't have that, an equivalent of silicon carbide is just as good. It takes time and might need tweezers to get the abrasive into the awkward spaces. Causes? Hardening of the nylon substance and minuscule growth of whiskers on the fins (on the SU) and various additives in the fuel can form semi-glues which prevent the fuel inlet valves on both carb types from fully closing.