Check that the float needle is not stuck in the underside of the float chamber lid, and that the carb pees petrol out of the vent hole in the float chamber lid after you have pressed the tickler button down for a few seconds. Also GENTLY screw in the idle mixture screw to its limit, and then unscrew it one and a half turns. Then try starting it again with air lever closed and throttle approx. 1/4 open. Don’t forget to turn ignition on. Good luck !
