Thanks aspeed. Yes, the tank couldn't be any closer to the engine on the plane and on the test bench.

I went ahead and vacuum and pressure tested the carb. The only leak is around the throttle barrel, which is normal. Put the engine back in the Champ, set the airbleed for a reliable 2500 rpm idle with the tail raised up to level. I guess I'm just going to have to live without being able to idle for more than 5 sec with the tail on the ground. I've never had this problem using larger engines with twin needle carbs but whatever, it still pulls the 2-1/4 lb. plane around nicely.

This will be my first and last OS airbleed carb engine on a tail dragger.