so since you have cleaned the carbs, you know for sure the pilot jet is free of debris,the pilot circuit also flows out the fuelscrew orfice in the throat of the carb when you spray carb cleaner thru the hole where the pilot jet screws in..right?.
so this leaves me these options to check next.
1..) the needle and seat in the carb are working correctly, and it shuts off the fuel? is the float level correct when you had the carbs off?
2.) swap the spark plug boot only to the next wire over and see if the problem follows the boot,sometimes the resistor gets weak in them and lower engine speed is less voltage from coils.
3.) inspect the reed block from that cylinder see if any petals are missing or chipped off
