There is one more thing worth checking, I found this problem on my ’49 DPY engine.
At some time in its life it seems someone tried to ‘improve’ the ports on this engine. As a result they broke through the crankcase casting from the inside edge of the bottom LH transfer port to the flywheel space. They then filled it (badly) with plastic padding or something like it. Time, oil & vibration took their toll and loosened up the plug allowing air to bleed in.
I found myself a top-notch welder who filled up the hole for me. After a brief visit to my milling machine for a refinish of the transfer port gasket face all was well.
The point here is that, at least in the case of my engine, the casting is pretty thin so not only is there a danger of over enthuastic port polishing but I have heard that casting porosity is not unknown.
It would therefore be worth the time spent to do a very critical inspection of all the intake tracts and mating faces.