I have seen such vehicles with CNG running around. The problem could possibly be due to a misfiring causing the backfiring.
September, 9, 2011 AT 12:56 PM
September, 11, 2011 AT 7:06 PM
Get some new spark plugs. NGK or Bosch will be fine. You'll want to get plugs that are one range COLDER than specified for a similar petrol-only engine and with a narrower electrode gap (0.8mm). This will provide increased driveability and resistance to backfire on LPG.
A good set of HT leads will also be of benefit as you'd be eliminating another common cause of backfire.