Is the throttle cable frayed or sticking? Possibly a bent needle hanging up. Have you ran it with the filter on it? Filter between the tank and carb? Seems like you've done a lot of inspection. The only thing I can think of is start back at the basics. Sorry, not much help I know but I have been in your shoes and usually find the issue when I start back at step one. Vaccum pressure in the motor, cables, dirty carb, leaking head gasket... Things like that. Best of luck.
81 300R
85 200x
85 200s
85 250sx x2
83 185
80 110
83 110
78 90
84 125m
Bandito 90
Dickson Full Suspension