Make sure you are using fresh fuel. If you leave the tool unused for a few months, you need to change the old fuel before you start using it. Secondly, in the muffler, there is a spark arrester screen that needs regular cleaning.  Thirdly, clean or replace the fuel filter in the fuel tank. Lastly, replace or clean the air filter. If all these solutions don’t work, take the product to your nearest authorized Stihl service dealer.