Well, filters don't need to be near the tanks... they can be just before the engine, for that matter.
One objection I might have is that soluble matter (if any) from the sealing bladders would get through the filters, then deposit in the carbs as the spray evaporated. But after 77 years, I reckon all the solubles (if any) in the bladders would have been carried away long ago.
If it isn't the fuel after all, I'm betting on the impulse magnetos. Maybe they ran out of impulse (weak springs?) and they can't quite give the same "snap."
Seems to me something like that would explain the shutdown, the fiddling with the engine, and the final "go" decision. And I think they said the port engines were overhauled, but not the starboard ones, no?
(Do you guys switch (test) the magnetos at idle, or at run-up RPM? I can't remember.)
Terry, groundlubber in a nice soft armchair, 230RN
REF (You can hear the clunk-clunk of the impulse magnetos in a lot of radial engine startups):
https://en.wikipedia.org/wiki/Ignition_magneto#Impulse_coupling(scroll to impulse coupling)