getting backfire

[jerry kingman <ter.jer@xxxxxxxxxx>]

     i'm getting backfiring from two of my cars. one is a 1985 323i that
had a catalytic converter welded in; these cars did not have cats on
them originally. the cat on this car seems to have something loose
inside. i tried taking this cat out and substituting an old one that i
had from an older car. the backfiring is a little less than before but
it is still there. the backfiring that i get is right after accelerating
and then backing off the pedal i get this noise. do i need to get a
complete new cat? i wish i could just put on a euro exhaust but i need a
cat for emissions.
    the other car i get backfiring is a 1980 323i with euro exhaust the
way it is suppose to be. i just put in a new stock motor and i get a
similiar condition but not as pronounced.
    i'm wondering if it's the motors or exhausts or both. isn't
backfiring unburnt gases?
any help would be appreciated.

        jerry kingman bmw cca# 93056
        1980 323i (for sale)
        1984 520i (for sale)
        1985 323i

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