I'm not extremely familiar with the anatomy of a B-29 (radial engines yes), but tracing the origin of the flames back in the picture I stole from the screenshots thread, they clearly seem to originate from within the actual engine cowling, escaping through the louvres? I assume the schematic is illustrating fuel pumps there?
I'm not trying to be argumentative but the flames in the screenshot do look like an engine fire, do they not? One can see at least why one might form that impression.
Thank you for the images, interesting.