Having problems with my old trimmer. Put new carb kit in the carb, new fuel filter, new duckbill & vent plug in gas cap. It will start and run great for a short while. Then it shuts off. When I try to restart, gas will run out of the carb but when I check the spark plug, it is dry. When it is setting there and not running, I can actually see gas siphoning up thru the primer bulb hose.