Backfiring T3

[Douglas Henry <dhenry@u.washington.edu>]

Hi all:

Since rolling out my '71 fuel-injected squareback from it's resto last
year I've had a little problem with backfiring.  I'm a little ignorant on
the causes of backfiring, and I was hoping for a little help on this.  It
typically happens in second or third gear when I am accelerating and
abruptly let off on the gas pedal to stop or pause for something.  I
didn't have this problem before; here is a list of possibly relevant
things that were changed when it was garaged:

1. New muffler system (stock, same as before) Can a loose fitting cause
this?
2. Position of fuel cut-off switch moved slightly. 
3. New points, condenser, plugs.

Is backfiring purely a timing problem, or can any of these changes
contribute?  My plugs, I should say,  are typically black when I change
them, I can't seem to get the timing right to avoid an over-rich fuel
mixture. 

Thanks in advance,

Doug H.
'71 Squareback (fuel-injected)