Agree,just one restriction in the rubber line at center carb, no mods. Drilling metering blocks is not needed if his afr is ok during normal center carb operations. My opinion only.

Not sure if following helps . But had similar issue with my 528RB. Heaviest springs from set in outboard are still too light for the venturi vacuum from centre carb.
Had a T-off in the hose with small air-bleed (0.6mm) to have the outboards open a little bit later when peddel was floored.
Had it first in the hose but caused outboards to close slower when lifting gas.

Mick, any info to update???? I'm thinking of going to 6BBL setup on my 472....

No 8.5 pv

Try one these too ..

https://board.moparts.org/ubbthreads/ubbt...tml#Post2550817

Met block