Small Engines (Lawn Mowers, etc.): mower backfires, valve clearance, model numbers

Question
12 hp b/s engine backfires through muffler and wont stay running changed spark plug and filters but didnt help if anyting got worse

Answer
Hello David:

This can be Caused by the Flywheel Key being Damaged or the Valve Clearance Requiring Readjustment. Send me the Model Numbers Off the Engine, Please. With these I can Locate the Proper Manual and Better Assist you. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John