I got my very first 6bbl because of the very issues being discussed here. A guy had bought a 4 year old 70 440-6 Cuda. Nice car, but was just old enough that the secondaries were inconsistent on their openings. Sometimes yes, sometimes no. None of us knew much of anything about that stuff back then. He messed with it, had others mess with it and soon enough, it was all messed up.

He became so frustrated with it that he offered to trade for the 340-4 in my 70 Cuda. My 340 was nothing special, only had a set of hedders, but it did run good. I really liked his set-up although I didn't know much about it. So we swapped everything in our cars, from the radiators to the trannies.

After a bunch of reading and experimenting, I did get the carbs to be more consistent. Added, cam, hedders, etc. Man, that thing was a monster. Street raced it for years. I even spun rod bearings and drove it home knocking. Pulled the pan and offending rod cap, polished the bearing off the crank and put it back together....in the car. Did that 2 or 3 different times.

But, like others have touched on, it had to have some traction (load on the engine) to get the secondaries to open.