I drilled out the air bleed hole with the smallest increment drill bit I had... 5/64". Reassembled and started the engine. Wide open throttle & midrange were still excellent. When I went to idle, I'd guess 2000 rpm with 15x10 APC prop, the engine would sag a couple seconds & then speed up a couple hundred rpm (might be the tiniest bit lean?) but then idle for as long as I cared to....
Additional tests confirmed results & gave me confidence in our fix.
Case closed! Just ordered a case of fuel... [8D]
Thanks Dave!