[E-Type] carb help

I’ve rebuilt my dual carbs (175 CD-2’s) on PrtyKty (1971 4.2). They
are working fine except under petal-to-the-metal acceleration. The rear
carb leans out to almost nothing.
Have I got a passage blocked? At a steady 3000 RPM it is fine, as well
as when I accelerate slowly. I pulled and cleaned the piston, checked
the diaphram and needle, can’t see anything.