Small Engines (Lawn Mowers, etc.): backfire through muffler, engine model type, mtd yard machine

Question
i have an MTD yard machine 1996 model with the 20 hp B&S twin on it . it's backfiring through the muffler when i shut it of can anyone help me out ?

Answer
Does the engine have an anti-afterfire solenoid on the carburetor?  When was the last time the valve clearances were checked?
What is the engine model, type and code number?

Have you read this link:

http://www.briggsandstratton.com/support/frequently-asked-questions/Inspecting%2

Is the carburetor running too lean?

Eric