midget backfiring

[Pheysey <pheysey@enol.com>]

My 78 midget is backfiring--not too badly, and generally when shifting
from first to second as the engine slows down a little.  It's not very
loud, but it's something I would prefer the car not do.  the car
seems to do it more often when under hard acceleration.  besides this
the car runs well.  I believe backfiring is from unburned fuel igniting
after it's left the engine?  what are some things I could check for or
try?  I don't know if this helps, but the CO levels were pretty high for
emissions test.
thanks much
STEVE PHEYSEY
78 Midget