The Spark Arrestor Screen is in the Muffler. It is Either Attached to the Outside of the Muffler or Inside the Muffler. If Mounted on the Outside, then you can Remove the Bolts Holding the Cover and Simply Remove the Screen and Clean it or Replace it with a New Screen. If it is Inside the Muffler and the Muffler is a Sealed Unit and Does Not Seperate, then a New Muffler is Required. If the Muffler will Seperate, then the Screen can be Cleaned or Replaced just like the Extrernal Spark Arrestor Screen.
You can View a Breakdown of this Unit at this Site Addy; http://www.ryobitools.com/parts
and Select "Go to Ryobi Outdoor Parts Store". Now Select "Repair Parts Catalog". Select I Agree to Terms. Now Enter the Model Numbers and Click Search. Now Double Click on you Model in the Results Area.
Please, Do Not Hesitate, If I Missed Something or you Hit a Snag or this Does Not Correct the Problem, I am Here if You Require More Assistance.
Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. Be Safe and Be Happy. Thanks.