Lift Throttle Backfiring

["Vincenti, Ross" <Ross.Vincenti@transamerica.com>]

Greetings Scions.........

Does anybody else experience this "problem"? (I put that in quotes because I 
actually kind of like the effect I am about to describe).  In my slightly 
tweaked Spitfire Mk I w/ 1147 motor, when I run it up fairly high on the 
tach (not to redline, just 3,500 or 4,000 or so), then lift off the throttle 
such as coasting to a stop light or behind heavy traffic (with the car still 
in gear mind you), I get a really nice terrific fat burbling sound with a 
couple of pretty good "pops" out the tailpipe (backfires).  (Scares the hell 
out of little old ladies and dogs,  and gets smiles from college coeds so I 
don't care to fix it frankly.)

Question - what causes this?  It sounds very much like some of the vintage 
race cars I have watched at various races, but I've never understood what 
causes it.  I don't think it is a "true" backfire where the charge gets 
blown back out of the carburetor (or am I wrong on this point?).  Does 
anyone else experience this?  Thoughts please,

Thanks.
Ross D. Vincenti
64 Spitfire 4
64 Porsche 356C Coupe