How can i repair my Talon 25cc trimmer will not idle?

Motor runs fine but will not idle on its own.
I have cleaned air filter & cleaned out carbi pulled apart & flushed.Could this be due to a worn spark plug or is there a modification to the carbi that can be done.Sometimes ok in summer months much worse in winter.Have had problem since new thought just needed to be run in.Its now 3yrs old.
Any ideas would be great!
Thanks

Tez
June 2011