pop and snap backfire on deceleration

[andy65 <andy65@snet.net>]

BJ8 - When I decelerate I often get loud pops and snaps and sometimes a
backfire that sounds like the crack of a rifle shot.  Only happens when
decelerating briskly and foot is off the pedal or  less than one quarter
throttle.  Plugs look normal to blackish.  Vacuum advance is not working.
Checked timing and it is ok.  Otherwise seems to run fine.  Where do I look
next?

Andy