My 40 FP is dead reliable with the air bleed all the way open. You could probably just take out the screw. All my old air bleed carb engines need to have the hole wide open, Enya or OS. I would open it all the way, and then try the test above.

While it's possible the plug is weak, I doubt it. Break-in can weaken plugs, but those engines don't need a lot of break in; mine were always fine after break in, and I've had a variety of FPs.

Not sure why it idles well on the ground but not in the air, but check the bleed hole.

Jim