Small Engines (Lawn Mowers, etc.): lawn tractor back-firing, craftsman lawn tractor, octane gasoline

Question
Hi,

 I have an old (late 80's) Craftsman lawn tractor that belonged to my father (I think it's a 12 horse power).  The last time I ran it (I was also pulling a lawn vacuum to get up leaves) to cut the grass and remove leaves, as soon as I parked it and shut it off it back-fired.  A spark came out of the exhaust pipe with gunshot-like bang.  I have heard of cars doing that, but I never had a car that did it nor do I even know what causes it in cars.

  The tractor runs OK, sometimes it is hard to start when it's cold, and I always check the oil and clean out the airfilter.  Do you know what would cause a small engine like this to back-fire?  
Currently I use low grade 87 octane gasoline in it (I have no idea what my father did with it or how well he maintained it, but he seemed to take decent care of the lawn equipment) - could that be the cause - or does it need new spark plugs?  

If you have any ideas what might be causing this and how I might be able to correct the problem I would greatly appreciate it.  My money situation is not good now and I really can't afford a new one and want to keep this one going as long as possible.

Thanks,

Lee

Answer
Hi Lee,

This is common when an engine is run hard and suddenly shut off while hot.  When the spark is stopped, unburned fuel goes into the muffler, where it ignites.  I usually let my mower idle for about a minute, then shut the key off.  

Let me know how you make out.  
Michael