I have a troy bilt weed eater, just a few years old. When it starts it runs fairly smooth until I squeeze the trigger, at which point it starts to die out. Sometimes right away and sometimes after a few seconds. I can't accelerate the trigger all the way else it dies out. Last week I replaced the spark plug and it worked fine right after that. I just tried it and it is doing the same thing as before I replaced the spark plug. Any suggestions would be greatly appreciated.