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Just logged in for a completely different problem (on the 1800 column), but my first impressions of the HS-4 Duett carb issue are based on what I see in your photos, and my own HS-4 experience.
1.) When they get dirty and gritty on the outside, then they also allow grit into the throttle shaft openings. The grit accelerates (pardon the word) the wear on those soft brass pieces, all that heating and contracting over the miles. Keep them clean.
2.) I agree that they need special circular return springs on the shafts themselves. The after market return spring does work, but will cause excessive wear if it's like huge garage door spring! Installing "improvements" isn't always a bad thing, but sometimes they do conflict with the original function, which won't be an improvement. The original parts in original functioning condition is always best. Remember, this is 1930's technology. Low-tech at its best.
3.) You didn't say if the dash pots were always filled, which really affects the idle and (mostly) the acceleration. I use ATF but you can pick you own (low viscosity) poison. You can even buy expensive SU oil from the British car places.
4.) Correct me if my vision is faulty, but it looks like there is a small rubber "o" ring on the throttle shaft, just under the idle adjustment screw of the rear carb. Good idea in theory, helps to seal any air leaks at the throttle shaft location. However, the resistance it provides as the thing gets hot, expands, and starts to disintegrate, will cause the shafts to bind, and,..you got it, lousy idle. There are creative ways to engineer the use of additional seals on these carbs, but they do require careful attention to the overall effect in operation. I have a German classic car magazine article which shows sealed roller bearings, specially machined shafts with Teflon seals, and all kinds of Teutonic zeal. I'm sure it works, if you wish to invest the effort.
5.) Consider buying a second set of HS-4 carbs, rebuilding them to original, or better specifications, then keeping them in reserve. Then, when you start having problems (and you will!)just swap them out. Minimal down time; half an hour, versus days, or weeks of down time.
6.) I know we shouldn't be using the BB to self-promote, but I do have BOXES of everything you could need, plus NOS (SU and Volvo) rebuild kits. 30+ years of being a Volvo-holic adds up!
Hope this adds something to the discussion,..
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