Have a look at Robinc's thread, concerning his "carb joints".
Posts #129 to #140.
I'm still reading....and I'm not sure where the "Pilot air port" is so I can try that test??? But, interestingly, robinc's experience somewhat mirrors what I'm seeing on the right side...
Left side
- Turn in mix screw fully, stalls.
- Return mix screw, block Pilot air port, stalls
- Left carb pilot circuit looks good
Right side
- Turn in mix screw fully, should stall but runs really well, no miss, stumble or climbing idle….huh
- Return mix screw, block pilot air port, slightly rougher running, idle increases
- Right carb pilot circuit looks like it’s still plugged somewhere
In my case, turning the screw in fully doesn't eliminate the climbing idle or backfiring, but it does change it. More importantly, unlike the left side (which stalls when the screw's turned fully in), the engine does NOT stall when the screw's turned fully-in on the right side. That's one of the first things I noticed when the flat-spot issue began....