i assume you are blocking the exhaust as you pull the pull starter ? if your not getting fuel flow check the tank seal.. open the lid and screw the little screw inside the lid a half turn.. thai will make a better seal..

Way to test is blow into the exhaust pressure line while blocking the other end and listen for air leaks.. you should be able to hold both ends for at least ten secods and then release the carby end and hear the air woosh out.. if it dosent do that you still have an air leak.. check the tank for cracks too .. and check the rubber seal on the exhaust at the back of the engine and the rubber coupler..