Check the little cork donut shaped gasket that goes between the carb body and sec vac diaphram. If it is missing or leaks
between the vacuum housing and carb body, the vaccum signal that opens the secs at WOT, will be too late.
It will not build fast enough.
Usually when things are working right, No vac leaks, the lightest white spring is way way too light causing the secs to open too fast.
Generally a medium silver ( plain), yellow or purple spring is best for performance use.