[6pack] Carburetor problems?

[John North]

When I floor it at fairly low rpms, the engine coughs and splutters.
Very embarrassing in the auto cross.   I have a K&N O2 sensor system
installed, it normally shows quite rich (2 green bars), but when I
floor it it goes to full lean until the engine recovers.   Timing is
spot on, valves adjusted, gaps and points set, distributor rebuilt,
new rotor, new wires.   Have swapped out the coil.   Carbs are
balanced and set to mid range mix - two full turns from full rich,
doesn't seem to make much difference where they are set, O2 sensor
still shows well on the rich side and the engine stumbles horribly.
Dashpot oil filled.

This is a 76 with all the smog control equipment removed.   9.5-1
compression, mild porting, Good GP2 cam

Any suggestions?

John North