TRX 300 fw 2000 wont idle

IM working on this ATV for a customer and i have to keep my thumb on the throttle to keep the idle. Ive cleaned the carb already and readjusted the pilot screw to factory still a little varnish in bottom of bowl Ive cleaned main jet slow jet and starter jet and shot carb cleaner threw all ports as best as possible i have the throttle stop adjusted all the way to open throttle plate as much as possible but still stalls at idle and back fires a little bit on decelerate when riding any ideal>?