Small Engines (Lawn Mowers, etc.): chain saw, spark arrestor, exhaust port

Question
I have a craftman chain saw that doing the following. When I pull the rope to start it, it catches. Seams as though it is backfiring. When I remove the spark plug it allows me to pull continously. when I put the plug back in I am unable to pull the rope. I checked the plug and has good spark. Never ran accross this problem before. help.

Answer
 Hello Otis:

 Check the Exhaust Port and Muffler for Blockage. Check the Spark Arrestor Screen Also. If these are Clear, then Check the Flywheel Key. Remove the Flywheel From the Engine to Properly do this. It May Look Good while the Flywheel is Mounted, but it is Sheared when the Flywheel is Removed. Hope this Helps. I am here if you Require more Assistance. Thanks.

 Good Luck

 Respectfully

 John