Having experienced myself the issue with needle not working as it supposed few years back I was thinking that perhaps a pressure test would be a useful practice. Something along these lines: disconnect carb input line from fuel pump, jury rig a bicycle pump with pressure gauge, build up 5 psi, observe how long it takes for the pressure to drop. This should also detect the dirt on the needle Hillbilly mentioned. One could also shake the motor vigorously to try and imitate the issue with needle seat unsealing due to vibration during operation.
And another simple check would be for a presence of gasoline at the bottom of air intake. After few minutes of idle, remove the airbox top, remove the foam airfilter, look straight down the intake. If it is wet with gasoline it is an indication of flooding likely due to one the issues Tom and Hillbilly pointed out.
