On cold start with the choke set, the engine starts normally, but after a few seconds it begins to bog down. I've found I can fix it by pulling and releasing either the choke rod (temporarily simulating hot manifold) or the choke pull-off linkage (temporarily assisting the vacuum kick). Either way, the choke is forced open a little bit and the idle comes up nicely. What's going on here? There is no binding in the choke valve; it moves freely by hand. I'd suspect the choke pull-off linkage is out of adjustment, but why would a temporary assist result in a lasting change to the choke valve position?

Because there is (by design) a great deal of friction, tension, and counter-tension in this system. Squeeze the U-bend in your choke pull-of link slightly, as described here.

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