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I've got to agree more with Walrus. On throttle-off, the butterfly will slam shut and if the thick oil in the dashpot does limit air piston drop speed, this will induce excess lean condition due to low air velocity and low vacuum at the main jet/needle location.

Don't understand delay on throttle off. Is your linkage free? Are you sure those vacuum leaks are minor? I'd certainly take care of the broken studs and gaurantee vacuum-leak-free manifolds before diddling on the carbs too much.

I think CDU **IS** on the right track with the springs. Increased spring tension will give a richer mixture and due to F=k*x the effect will be more pronounced as the piston rises. Also, I believe that SUs and Strombergs achieve piston full up well in advance of WOT. Consider how much piston rise you can get by just blipping the throttle and inducing high RPM with no load. After the piston is full up, the venturi section and effective jet orifice are fixed and operate like a "normal" carb.

You may consider a Colortune, which answers a lot of questions about mixture at idle and part throttle. Well worth $50, IMHO.

Later,
Al