Last post got me thinking about the breathing system.
I take it your car has a full, complete breathing system?
Excuse me while I talk myself through this. It's as much for my memory as for anyone else!
So, 4 small hoses from the nipples on the inlet manifolds go to a plastic block. This then splits of into 3 larger hoses. 1 goes to the oil filler neck and the other 2 to the air box metal bend nipples.
With the throttles closed, crankcase pressure/oil mist is relieved through the filler neck, small hoses to the inlet manifold nipples (below butterflies).
When the throttles are open, crankcase pressure/oil mist is relieved through the filler neck, large hoses, through the carbs.
I know you have replaced all of the breathing system hoses etc, but have you eliminated it all together?
My car ran on trumpets, with a breather filter straight of the filler neck. All of the hoses were gone and the inlet manifold nipples capped (with brake caliper bleed nipple rubber caps).
It may be some cylinders are breathing, and some not. On the twin cams, the idle breathing is only through cylinder No. 1, not all 4. I always plug this when doing balancing/idle mixture checks.
I'm not suggesting binning the breathing system permanently. Just to try capping the inlet manifold nipples and then doing the full set up again as described earlier. It might show up a blockage in some nipples or a hose.