OUCH, sounds like it's getting very lean in some spots and is possibly already damaging the motor, could be an air leak. I'd do a full checkup and apply oxygen sensor safe silicone to the carb neck gasket and back plate. Also ensure the middle speed needle (opposite of throttle arm side) is flushed and untouched, I once accidentally was turning it thinking I was doing the LSN! :stupid:

I am an ex-mechanic as well and I thought my experiences would pay off in these little nitros, but they're quite different- I got stumped a few times myself.