BTW, it takes what? 30 seconds? to shut down an engine, turn the idle needle 1/8 turn, and crank back up?

Yeah, there are a couple of important details.
Don't hamfist the idle needle. It's a fine adjustment device. 1/8 turn is good for each test. If the first test isn't obvious, do 1/8 and retest again until it is obvious.
And always open the throttle to do the low needle adjustment. Matter of fact, look into the venturi to see if you can see the needle while you're at it.

I've seen too rich idle needle settings kill the engine in the air. I've seen too lean idle needle settings kill the engine in the air. The symptoms overlap. But testing on the ground won't be fooled. And won't take any time at all.