Small Engines (Lawn Mowers, etc.): Big bang, gas vapors, pro mechanic

Question
Dear sir,
  I am perplexed. I have been working with gasoline and diesel engines since I was 16,(I'm now 27), I have never had an engine backfire when shut down as this one does. I have checked the Timing, and the Mixture. Repaced the points , Condenser, and checked the dwell... I even replaced the air filter. It is a very small 98 CID ford four cyl. In a 639 Bobcat. I also have a Cub cadet Model 129 Hydro with a kohler K301A engine, That does this on occasion... the cub never bothered me so I simply paid it no mind.
   Please advise I'm stuck,(Pretty sad for a "PRO" Mechanic Huh)?


Answer
 Hello Carl:

 Change to a Different Type Muffler. The Muffler is Trapping Unburnt Gas Vapors. Let the Engine Idle for a Minute or two before you Shutoff the Engine. This will Help Clear the Muffler. Also this is a Characteristic of this Engine in the Bobcats. Like you I tried All the Usual Remedies and none have Worked on Any of the Engines. The Muffler and Idle Time before Shutoff does Seem to Help on some. I am here if you Reqire more Assistance. Hope this Helps. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John