I would start with removing the mixture screw and jet and cleaning them. Squirt some carb cleaner into the body of the carb and chase it with compressed air, easier to see whats going on with the carb off the motor, like Grumpy has suggested, clean everything all at once.
Check the end of the mixture screw for marks. Ive got a whole heap here that have been over tightened, they get a ring around the needle and it is then impossible to adjust the mixture as the needle is lean or rich with no in between.
Another possiblity would be a leak in the inlet tract, its a gasket on that motor i believe.
