Hello,
Thank you for the replies.

I have already replaced all the 26 year old rubber fuel lines and blown out the metal lines to make sure there is no blockage anywhere.

I am suspecting two areas now:

1. The fuel pump drive cam may be worn out and causing the fuel volume in the carb float bowl to drop far enough to cause a lean out at idle after running at high speed.

2. The vacuum fuel return valve is overenthusiastic and returning too much fuel at speed, though the car easily pulls 90-95mph with the a/c engaged in 90-105F, with engine temp holding around 90C. I am thinking of rigging up a vapour separator canister(Solex carbs in the 1990s had these) and disable the vac return valve, this has a simple restriction in the fuel return line, just to see what happens.

As Peter suggested, I have it running slightly rich and even when the idle is rough, the car will pull away from a stoplite without any problem, but thats because the Stromberg uses only one jet for ALL operating conditions. I have always been able to sort out carbs, even the Solex 4A1 is no real mystery to me, but this one has me stumped.
Under 85-90% of my driving conditions, the car more than impresses passengers with it's glass smooth idle, good performance and of course the ability to smooth out the worst of badly worn out and potholed urban roads, but this poor idle bit has really got me.

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Nachi11744