I have a 4 cycle craftsman trimmer purchased about 6 years ago. I recently had trouble getting it to start. I replaced the fuel line, fuel filter, and gas as I have had to do in the past but it did not work this time. I then replaced the carb. Still would not start so I replaced the spark plug. Now it will start and idle just fine but as soon as I give it some throttle it dies. I read around and some mentioned the possibility of the gas cap vent being plugged so I replaced it this morning. Same problem. Any ideas of what to check next?