How about the choke pull off diaphragm? It might have a small tear in it an not holding vacuum? (working)
On cold startup the choke flap is fully engaged/closed for maximum richness to allow easy cold starting. Once the engine starts, engine vacuum will be applied to the choke pulloff and its plunger will be retracted, opening the choke a little for a less rich mixture to keep engine running properly.
The pulloff plunger also controls the secondary air valve opening rate. When wide open throttle (WOT) is applied, the engines manifold vacuum is reduced significantly and the pulloff begins a controlled release of its plunger. This controlled release is what partially controls the opening rate of the secondary air valves.