Backfiring sud

[Duncan Galloway <Duncan.Galloway@xxxxxxxxxxx>]

Hi guys,
  I'm in a pickle. My long-suffering 1978 Alfasud TI is beginning to
backfire terribly. I got it tuned & serviced about 5 months ago (it is
perhaps due for another one) and up until a month or so ago it has been
fine. Now it is backfiring, but only when the engine is revving high but
not under any load. That is, if I'm going down a steep hill in 3rd and I
take my foot off the accelerator, lots of bangs. Any other time, no
backfiring. Otherwise it's running really well.
  Can anyone suggest a remedy for this? Could it just be the mixture
screw on the carby or something like that? From what I understand
backfiring is from unburnt petrol igniting in the exhaust system, so I
can't really work out why this would be happeing with my foot off the
accelerator (=only a little bit of fuel going into the motor).
  Thanks in advance,
Duncan

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