Running-On, Etc.

[gnh.wbst207v@xerox.com]

Given a propensity for some lbc to run-on, what explains a 
phenomenon common to both my MGA and TR4A:
  
A few seconds after shutting off either engine, they backfire out 
the tailpipe once.  I'm never back there to see if there is flame, too.  
Both engines have increased compression, headers, some additional 
static advance and use 93 octane fuel.  Neither engine runs unusually 
hot.  Both run-on occasionally and then don't backfire.  
Should I be concerned?  Any ideas?

George