Engine Exhaust Backfire 

1978 Chevy Luv, 56k miles, backfires  about 80% of the time when
engine is turned off. It's very loud, and comes out of the tail pipe. 
Otherwise, it runs fine.  What are the likely problems and solutions?

I assume that this vehicle has a v-6 (I can't remember).  could be a
few things.  The timing could be way off, is it hard to start?  Does
it run after the ignition has been turned off?  Basically, what is
happening is fuel is moving through the cylinder chamber, not getting
burned and igniting in the exhaust manifold.  So from there, you could
also have a stuck valve or bad rocker arm.  Does the vehicle have a
miss?

Thanks Shockwaveracing,
It's an inline 4.  It starts OK and runs fine, although in the winter
the choke sometimes sticks...  Haven't noticed any missing.  Thank you
for the response.

In addition to checking the timing, another likely possibility
is that the fuel mixture is too rich. While this might be 
adjustable, it's also possible that the carburetor needs an
overhaul, with particular attention to the float valve.
A rich mixture can be detected by smelling the exhaust (briefly!).
If it smells of unburnt, raw gas to some degree, this is 
your problem.

Boy, without looking at the truck, it is tough. So, let's look at what
happens.  Somehow, unspent fuel is entering the exhaust manifold,
igniting the fuel. At only 56K, it does not seem likely that the valve
seat would be worn out. And if you have not noticed any type of engine
problems, then that should rule out a bent or burnt valve.  When was
the last time the spark plugs were changed?  A tune up could be the
simple cure, but again, without looking and actually having the
vehicle right in front of me...........